In a frank, 30-minute discussion on Sunday night, Mr. Obama urged China to put the North Korean government on a tighter leash after a series of provocations, most recently its shelling of a South Korean island, which has stoked fears of a wider military confrontation in the Korean Peninsula.

Mr. Obama, the official said, told Mr. Hu that “it was important for the North Koreans to understand that their actions would have consequences, including in their relations with China.” He reminded the Chinese leader of a tense meeting they had in Canada last June, after which the president publicly declared that China was guilty of “willful blindness” to North Korea’s military provocations.

Since then, North Korea has lobbed artillery shells at South Korea, killing four people, and disclosed the existence of a clandestine uranium enrichment complex. Still, China, North Korea’s most powerful ally, has not spoken out against the government. Even Mr. Obama’s phone call with Mr. Hu took several days to set up, though the White House insisted that it was a scheduling issue, not an attempt by China to duck the president.

Mr. Hu did not offer any specific assurances to Mr. Obama, the official said, but he also did not complain about joint American-South Korean military exercises in the Yellow Sea. Nor did he suggest that the United States was partly to blame for North Korea’s belligerence because of its unwillingness to negotiate with Pyongyang.

“The call was meant to be more forward-looking than pointing fingers at the past,” the American official said.

Mr. Obama’s pressure on China was reinforced by a three-way meeting of the United States, Japan and South Korea at the State Department on Monday, at which they resolved to mount a united response to North Korea. The show of solidarity underlined the isolation of China, which has clashed recently with both Japan and South Korea, even as its relations with Washington have chilled over North Korea and other issues, including trade, currency squabbles and climate change.

In a coordinated series of actions, the White House is dispatching Adm. Mike Mullen, the chairman of the Joint Chiefs of Staff, and a Pentagon delegation to South Korea and Japan to discuss ways to deter North Korea militarily. A high-level delegation from the State Department will travel to the region next week.

“We would hope that China would work with us to send a clear, unmistakable signal to North Korea that they have to demonstrate a seriousness of purpose,” Secretary of State Hillary Rodham Clinton said at a news conference afterward, flanked by the foreign ministers of Japan and South Korea.

Still, it was clear that the United States and China viewed the North Korean threat from different poles — the Americans convinced that it cannot be allowed to act with impunity; the Chinese fearful that a strong response would either cause the government to collapse or provoke it to even greater acts of violence.

The United States has rejected China’s proposal for emergency talks of the six parties that typically meet to discuss North Korea’s nuclear program. Mrs. Clinton said such a meeting did not make sense until the North Korean government took “concrete steps to change its behavior.” The South Korean and Japanese foreign ministers agreed, and echoed Mrs. Clinton’s call for China to take a more active role.

“We would like China to have a clearer stance in giving warning to North Korea and to contain the provocative actions by North Korea,” Kim Sung-hwan, the South Korean foreign minister, said in Washington.

The United States, analysts said, is taking its lead partly from South Korea, which under President Lee Myung-bak has mixed tough words with restrained action in responding to both the artillery shelling and the earlier sinking of a Navy warship, the Cheonan, which killed 46 South Korean sailors.

“It almost shows that there is a new cold war in Asia, with Japan, the United States and South Korea on one side, and China and the North on the other side,” said Victor Cha, a special envoy to North Korea during the Bush administration.

Mr. Cha said the flurry of American diplomatic activity “sends a message to China: you can either stand with us, or you can stand with the North. That puts some subtle, or not-so-subtle, pressure on China.”

Whatever the reason for the time it took to set up the presidential phone call, analysts in Beijing and Washington say it epitomizes the speed with which relations between Washington and Beijing have plunged into a freeze. This year has witnessed the longest period of tension between the two countries in a decade. And if anything, both sides appear to be hardening their positions.

“The Chinese now feel broadly that where they disagree with the U.S., they can really say ‘no,’ ” said Kenneth G. Lieberthal, a China policy adviser in the Clinton administration who is now at the Brookings Institution.

In Sunday’s call, according to a White House statement, Mr. Obama told Mr. Hu that North Korea’s new enrichment facility flouted commitments it made during the six-party talks on curbing its nuclear program, and urged China’s help in sending “a clear message to North Korea that its provocations are unacceptable.”

One former Chinese official with close ties to the government dismissed the American approach last week as characteristically legalistic. The former official, who spoke on the condition of anonymity because he was not authorized to speak on the topic, said China’s strategy was to reassure the North Koreans about their security, not lecture them about diplomatic obligations.

Indeed, China’s strongest public reaction to last month’s shelling was not to condemn the North, but to criticize Washington’s response — joint war games with South Korea that put the American carrier George Washington and its strike force in the Yellow Sea, off China’s borders.

After Mr. Obama’s national security team met last Tuesday night, administration officials began saying that the United States would conduct more military exercises near North Korea and China should the North engage in further provocations. It was an unmistakable message to Beijing that failing to rein in its ally would only increase an American military presence that China loathes.

The trilateral meeting in Washington sent another message, according to officials: that the United States will work more closely with its historic cold war allies in Asia to compensate for the rise of China.

“There is a lot at stake,” Mrs. Clinton said, “and we are committed to working through all the challenges we face together.”

David Sanger contributed reporting from Washington, and Michael Wines from Beijing.