WASHINGTON » Intensifying sectarian and clan violence has presented new opportunities for jihadist groups across the Middle East and raised concerns among U.S. intelligence and counterterrorism officials that militants aligned with al-Qaida could establish a base in Syria capable of threatening Israel and Europe.

The new signs of an energized but fragmented jihadist threat, stretching from Mali and Libya in the west to Yemen in the east, have complicated the narrative of a weakened al-Qaida that President Barack Obama offered in May in a landmark speech heralding the end of the war on terrorism. The leaders of the Senate and House intelligence committees, Sen. Dianne Feinstein of California and Rep. Mike Rogers of Michigan, raised warnings in an interview on CNN on Sunday when they said that Americans were "not safer" from terrorist attacks than in 2011.

The concerns are based in part on messages relayed this year by Ayman al-Zawahri, al-Qaida’s overall leader, indicating that he views Syria — where the number of jihadist rebels and foreign fighters is steadily rising — as a promising staging ground.

Some analysts and U.S. officials say the chaos there could force the Obama administration to take a more active role to stave off potential threats among the opposition groups fighting against the government of President Bashar Assad. But striking at jihadist groups in Syria would pose formidable political, military and legal obstacles, and could come at the cost of some kind of accommodation — even if only temporary or tactical — with Assad’s brutal but secular government, analysts say.

"We need to start talking to the Assad regime again" about counterterrorism and other issues of shared concern, said Ryan C. Crocker, a veteran diplomat who has served in Syria, Iraq and Afghanistan. "It will have to be done very, very quietly. But bad as Assad is, he is not as bad as the jihadis who would take over in his absence."

It is not clear whether or when the White House would be willing to make such an abrupt shift in approach after years of supporting the Syrian opposition and calling for Assad’s ouster. It would certainly require delicate negotiations with Middle Eastern allies who were early and eager supporters of Syrian rebel groups, notably Saudi Arabia.

One growing source of concern is the number of Muslims from Western countries who have gone to fight in Syria and might eventually return home and pose a terrorist threat. Analysts say at least 1,200 European Muslims have gone to Syria since the start of the war to join the fight, and dozens of Americans.

Across the region, a rising tide of Islamist militancy — fueled partly by sectarian violence and partly by the collapse of Egypt’s Muslim Brotherhood in the face of opposition from the country’s military — has contributed to a recent wave of attacks, including bombings in Lebanon and the Sinai Peninsula as well as the daily carnage in Syria and Iraq.

The violence has underscored the continuing disarray across the Middle East in the wake of the 2011 Arab Spring uprisings. Above all, it is the chaos of Syria, where foreign jihadis appear to be building to a critical mass and have overwhelmed the Western strategy of support for the moderate opposition, that could drive the Obama administration toward greater involvement, analysts say.

But it is not at all clear what form that involvement might take. The use of drones is one option, but not the only one. In early October, U.S. commandos carried out raids in Libya and Somalia aimed at capturing terrorism suspects. One of those raids was successful, the other not.

To some extent, infighting among the jihadist groups in Syria has recently mitigated the threat there, but it is not clear how long that will last. Zawahri sent an envoy, Abu Khalid al-Suri, in an effort to resolve disputes between the two main factions, the Nusra Front and the Islamic State of Iraq and Syria.

"To the extent that I am concerned about al-Qaida the brand, it’s that it is clearly expanding its affiliates, both in number and in some cases in capability," Gen. Martin E. Dempsey, the chairman of the Joint Chiefs of Staff, said in an interview. "We’ve got to watch and determine which ones are local, which ones are regional, and which ones are global, and each requires a different approach."

Those agendas can easily overlap and change, and one place where that appears to be happening is Yemen, the home to al-Qaida’s most organized and threatening affiliate. A series of clashes in the past month between Zaydi Muslim militia fighters and hard-line Sunnis in Yemen’s remote northwest has led to calls for a wider religious war, and there are reports of training camps being established for that purpose, Yemeni officials say.

In Yemen, as in Syria, this sectarian dynamic may appear to divert the militants’ attention away from the West. But the accompanying radicalization and militancy creates "the perfect environment for al-Qaida" in a country where the terrorist group already has a strong foothold, said one Yemeni official who spoke on the condition of anonymity because he is not authorized to comment on the matter.

Even as a U.S. drone campaign continues to kill suspected militants in Yemen, the al-Qaida affiliate based there gained at least $20 million in ransom payments this year from the governments of Qatar and Oman, which paid to free two groups of European hostages, according to U.S. and Yemeni officials. That is enough to fuel their operations for years, the officials said.

A string of recent deadly attacks on Yemeni military targets has also made clear that al-Qaida "has infiltrated our security services" to a greater extent, the Yemeni official said. In one of those attacks, a band of six jihadists disguised in army uniforms commandeered a military post with dozens of soldiers inside and held it for three days, repelling repeated efforts to free the men.

In addition to the rising number and deadliness of attacks, there are signs of possible cross-pollination among some of the jihadist groups around the region. U.S. officials say that the Yemen-based al-Qaida affiliate, al-Qaida in the Arabian Peninsula, has regular contact with jihadist groups in Lebanon and in the Sinai Peninsula, where there have been near-daily attacks since the Egyptian military ousted the Islamist president Mohammed Morsi in July.

Despite extensive Egyptian military efforts to confront them, the Sinai militant groups remain strong and have powerful new weapons — including surface-to-air missiles that could take down airliners — obtained from Libya after its civil war, said Ehud Yaari, an Israel-based security analyst for the Washington Institute for Near East Policy.

The disarray in Libya, where the weak transitional government is largely hostile to the nation’s fractious militias, is also a source of increasing concern. Terrorism analysts say southern Libya has become a haven for a range of jihadists. "All of our regional partners are very afraid of the instability they see emanating from southern Libya," said Maj. Gen. Patrick J. Donahue II, the commander of U.S. Army forces assigned to Africa.

And other extremist groups are redoubling their efforts across Africa. Last month the U.S. State Department branded Boko Haram, the homegrown Islamist insurgent movement in Nigeria, as a foreign terrorist group. Its attacks have left thousands dead in a decade, and in October it ransomed four French hostages for a reported payment of over $27 million.

"Whether they are dismayed by the way things played out in Egypt or by the growth of al-Qaida in Syria, the worm has turned in the Middle East in the minds of American foreign policymakers," said William McCants, an expert on jihadist movements and a former senior adviser at the State Department. "It seems we are back to counter-terrorism as a guiding focus for American policy."

© 2013 The New York Times Company