THE HAGUE, Netherlands  >> With no sign of Russia abandoning the Crimean Peninsula, President Barack Obama said Tuesday he’s concerned that Moscow will move deeper into Ukraine and warned Russian President Vladimir Putin that that would be a bad choice.

Obama stood fast on his insistence that Crimea remains a part of Ukraine, even as the fledgling Ukrainian government in Kiev ordered its troops to pull back from the disputed territory.

"We’re not recognizing what is happening in Crimea," Obama said at his first news conference since Russia moved to annex Crimea after a referendum 10 days ago. Obama rejected "the notion that a referendum sloppily organized over the course of two weeks" would "somehow be a valid process."

Obama said he didn’t think international recognition of Crimea as part of Russia is "a done deal." But he also said, "It would be dishonest to suggest there is a simple solution to what has already taken place in Crimea," where Russia troops are in control.

"We also are concerned about further encroachment by Russia into Ukraine," Obama said at a joint news conference with Dutch Prime Minister Mark Rutte.

"I think that will be a bad choice for President Putin to make," Obama said. "But ultimately he is the president of Russia, and he’s the one who’s going to be making that decision."

Obama was pursuing efforts to pressure Russia out of its aggressive pose as world leaders met for an international Nuclear Security Summit. But to the east, the Russian annexation of Crimea was beginning to take root and Moscow shrugged off Obama’s drive to leave Putin in the cold.

Obama also said he was concerned about Russia’s troop build-up along the Ukrainian border. "We oppose what appears to be an effort at intimidation," Obama said. "But Russia has a right legally to have its troops on its own soil."

Asked whether in hindsight he agrees with Republican presidential challenger Mitt Romney’s assessment that Russia is the United States’ top geopolitical foe, Obama said he is more concerned about a nuclear bomb in Manhattan than in Russia.

"America’s got a whole lot of challenges," Obama said. "Russia is a regional power that is threatening some of its immediate neighbors, not out of strength but out of weakness."

The U.S. and some of its closest allies cut Russia out indefinitely from a major coalition of leading industrial nations and canceled a summer summit Russia was to host in its Olympic village of Sochi. Obama also sought to win backing from other foreign leaders in hopes of ostracizing or even shaming Putin into reversing his acquisition of Crimea and backing away from any designs he might have on other Eastern Europe territory.

In a strongly worded joint statement, the United States, France, Canada, Britain, Germany, Italy and Japan denounced a referendum in Crimea to secede from Ukraine and Russia’s ensuing annexation. In so doing, the seven leaders also effectively excluded Russia from what had been a two-decade-old coalition known as the Group of Eight.

"This clear violation of international law is a serious challenge to the rule of law around the world and should be a concern for all nations," the declaration said.

Still, Monday’s international gestures in Amsterdam and in The Hague got only a dismissive reaction from Russian Foreign Minister Sergei Lavrov.

"The G-8 is an informal club," he said. "It has no membership tickets, and it can’t purge anyone by definition."

Obama also raised the issue with Chinese President Xi Jinping. White House aides later commended the Chinese for refusing to side with Russia, a longtime ally, on a U.N Security Council vote last week declaring the secession vote illegal. Russia, a Security Council permanent member, voted against it, while China abstained.

And in an addition to his public schedule, Obama sat down with Putin ally President Nursultan Nazarbayev of Kazakhstan. As Obama and Nazarbayev wrapped up their meeting, the White House released a joint statement from Obama and Nazarbayev that did not address the Ukraine situation, but focused instead on bilateral cooperation on nuclear security and nonproliferation — the theme of concurrent summit serving as the official purpose for Obama’s visit to the Netherlands.

Obama praised action at the summit, including new commitments by Japan, Italy and Belgium to reduce their stocks of nuclear materials. Obama began the nuclear summit series in 2010 in an attempt to secure materials and keep them out of the hands of terrorists. Obama said the next summit, in 2016, will be held in his hometown of Chicago.

Later Tuesday, Obama was to meet with Prince Mohamed bin Zayed, crown prince of Abu Dhabi, the richest emirate in the United Arab Emirates federation. He also has a joint meeting scheduled with Japanese Prime Minister Shinzo Abe and South Korean President Park Geun-hye.

Both those meetings are likely to focus less on Ukraine and more on regional tensions in the Middle East and in Northern Asia. The visit with the Abu Dhabi crown prince will also serve as precursor to Obama’s on Friday visit to Saudi Arabia, where he will meet with King Abdullah to address Arab anxieties over the Syrian civil war and U.S. nuclear talks with Iran, a Saudi Arabia rival in the region.

The meeting with Park and Abe brings together two U.S. Asian allies who have been quarreling over recent Abe gestures that have rekindled memories of Japan’s aggression in World War II. It will be the first meeting between the two Asian leaders since they took office more than a year ago.