SANAA, Yemen >> Nine months of war between a Saudi-led military coalition and a Yemeni rebel group have left thousands of civilians dead, a nation gravely polarized and the land strewn with debris, mines and unexploded bombs.

The conflict has produced another bitter legacy: a new branch of the Islamic State that has quietly grown in strength and appears determined to distinguish itself as Yemen’s most disruptive and brutal force, carrying out attacks considered too extreme even by the country’s branch of al-Qaida.

The Islamic State’s deadliest attack, on mosques here in the capital, killed more than 130 people and helped start Yemen’s civil war in March. Now, as mediators are struggling to end the conflict, the group is fueling new tensions by carrying out powerful car bombings in southern Yemen and releasing videos filled with grisly executions and sectarian denunciations of Yemen’s Shiite minority.

Like Islamic State affiliates in Egypt and Libya, the Yemeni group has shown signs it is more closely coordinating its activities with the headquarters in Syria, analysts said. And its emergence has only added to the peril from Sunni extremism in Yemen, already home to a powerful branch of al-Qaida that has been able to seize territory during the latest conflict, including Al Mukalla, the country’s fifth-largest city.

U.S. intelligence and counterterrorism analysts say the Qaida affiliate, al-Qaida in the Arabian Peninsula, remains the most urgent militant threat in this fractured country. But they are closely watching the effort by the Islamic State, also known as ISIS or ISIL, to peel off defectors from al-Qaida’s wing here.

“The pace of its attacks and declarations of new provinces during the past year underscores the group’s ambitions in Yemen,” said a U.S. counterterrorism official, referring to the Islamic State. “While some may not consider ISIL’s Yemen affiliate to be as worrying as the group’s other hubs, there are a number of factors that indicate the branch should be taken seriously in the long term.”

An analyst in Yemen who closely follows Sunni extremist groups in the country said the scale of the attacks by the Islamic State showed that it was becoming just as dangerous as al-Qaida. At the start of Yemen’s civil war, the Islamic State’s presence was “limited,” said the analyst, who spoke on the condition of anonymity because of the hazards of talking openly about the group.

As the war has spread across Yemen and the violence intensifies, the group’s “recruiting circle is expanding,” he added.

Both the Islamic State and al-Qaida have profited from a security vacuum while trying to rally Yemen’s Sunnis against the Shiite-led rebels, known as the Houthis, who are from the north, analysts say. Crucially, the groups have both faced little or no resistance from the Saudi-led coalition and its allies, which are focused on defeating the Houthis. The coalition receives backing from the United States and Britain.

This month, Qaida militants were able to capture two towns in southern Yemen with little effort, residents said. In some cities, including Aden and Taiz, small numbers of hard-line Sunni militants continue to fight alongside the Saudis and their allies.

At the same time, Yemeni officials with the government of President Abed Rabbo Mansour Hadi, which is backed by the Saudi-led coalition, have appeared to underestimate the threat posed by the Islamic State — or even deny its existence.

Last week, when the Islamic State claimed responsibility for killing the provincial governor of Aden and eight of his bodyguards with a car bomb — releasing both a statement and photographs of the attack — the city’s security director, Mohamed Mousaed, insisted that “remnants of the Houthis and Saleh” had carried out the bombing. Yemen’s former president Ali Abdullah Saleh is allied with the rebels.

Nadwa al-Dawsari, a Yemeni analyst and nonresident fellow at the Project on Middle East Democracy in Washington, said there was a widespread perception in southern Yemen that the threat from the Islamic State was “manufactured.” That perception was fueled by the group’s “invisibility” as well as Saleh’s well-documented history of manipulating extremist groups for his own ends, including to win financial and military support from the United States for counterterrorism operations, she said.

But whether the Islamic State has grown organically or not, “the threat is real and the threat is strong,” Dawsari said.

The group has sent suicide bombers to attack mosques in Sanaa, which is controlled by the Houthis. In the past few weeks, the Islamic State has claimed responsibility for car bombings in Aden, including an attack on a hotel hosting members of Hadi’s government and another at a headquarters for the Saudi-led coalition.

A video released recently by the branch underscored its determination to showcase its brutality. In one section, the video shows masked gunmen leading prisoners to a small boat that was set out to sea and then blown up. Another vignette showed four captives made to wear what appeared to be mortar shells, draped around their necks, then pose for the camera before the shells were detonated.

The governor of Aden, Jaafar Mohamed Saad, was the highest-ranking official killed by the group since the emergence of the Islamic State in Yemen about a year ago. The relative ease of the attack showed a failure “to establish security in the south, despite a huge opportunity,” Dawsari said.

The Saudi-led coalition sent thousands of troops to drive the Houthis out of Aden and other southern provinces in July. But afterward, a decree issued by Hadi to integrate local resistance groups into the armed forces was never carried out, Dawsari said.

“The presence of foreign troops hasn’t been helpful, except for keeping Saleh and the Houthis from coming back,” she said. “Aden does not need foreign troops for security. It needs local security structures.”

The struggles by the coalition to establish security and stem the growth of the militant groups could carry consequences beyond Yemen’s borders, according to security analysts.

“The assassinations and other violence show that ISIS has gained power,” said Matthew G. Olsen, a former director of the National Counterterrorism Center in Washington. “There’s real concern that some of the most capable operational terrorists, who are now with AQAP, could join forces with ISIS and pose a heightened threat to carry out external attacks,” he said.

Others said defections from al-Qaida were gathering steam. “There are large numbers of both leaders and individuals joining the Islamic State,” said a former member of al-Qaida in the Arabian Peninsula who has left the group but remains close to its members. Many of the defectors, he said, were hard-line jihadis and young militants frustrated with al-Qaida’s failure to be more aggressive during the current war.

But the militants of the Islamic State, he said, “follow their words with actions.”