(CNN) -- The United States has "deep concerns" over China's newly declared East China Sea air defense zone, Vice President Joe Biden privately told Chinese President Xi Jinping during his visit to Beijing on Wednesday, according to a pool report citing senior administration officials traveling with the delegation.
According to the pool account of a background briefing by the officials, Biden also told Xi that the United States doesn't recognize the zone -- which includes uninhabited islands that have been the site of tense disputes between China and Japan, both of which claim the lands for their own.
"President Xi was equally clear in laying out their view of the zone and of territorials disputes in the region," one of the officials said, according to the pool report. "Ultimately, President Xi took on board what the vice president said. It's up to China, and we'll see how things will unfold in the coming days and weeks."
Biden had assured Japanese leaders on Tuesday that he would raise the issue with Xi, but it was not mentioned publicly by either leader.
In public remarks welcoming Biden that betrayed no hint of the tensions, Xi called the former senator an "old friend" and praised U.S.-China relations.
"We're happy to see that in the recent period, our two teams have worked actively to expand coordination and collaboration on bilateral, regional, and global levels," President Xi said in greeting Biden on his latest visit to China, according to a pool report.
Biden also made no public mention of the controversial Chinese air defense space declaration, instead focusing on the possibilities of good relations between the two nations.
He cited discussions between Xi and President Barack Obama this summer seeking a "new model" in relations between the countries as China seeks to expand its influence worldwide and as the U.S. pivots its attention from the Middle East to Asia.
"As we've discussed in the past, this new model of major-country cooperation ultimately has to be based on trust, and a positive notion about the motive of one another," Biden said.
"The relationship that you and President Obama have established thus far is full of promise, and real opportunity for us," Biden told Xi, according to a pool report.
"If we get this relationship right with a genuine new model, the possibilities are limitless," he said.
But in a sign the two leaders had much to talk about privately, a scheduled 45-minute private session between Xi and Biden ran two hours.
The meeting covered "every single topic in the U.S.-China relationship," including economic issues and North Korea, said one of the U.S. officials who were briefing reporters .
"They talked at some length about what the Iran example means for North Korea," the official said, referring to the recently reached deal between Iran and a coalition of international powers to curb the Middle Eastern nation's nuclear program.
The official was referring to how the international community came together to pressure Iran to reach a deal, according to the pool report.
Biden's visit to China comes about two weeks after China declared an air defense zone over a portion of the East China Sea, including island territories that China and Japan each claim.
The dispute over the islands has led to tense situations between ships and planes from the two countries over the past year, raising concerns that an incident in China's newly declared defense zone could spiral out of control.
It has also fueled concern among U.S. officials about how far China is willing to go to pursue its interests in the Asia-Pacific region and push back against U.S. influence.
China is asking aircraft entering its air defense zone to identify themselves and submit flight plans.
It says the zone is similar to others maintained by nations around the world, saying that it is not asserting territorial control over the airspace and that legitimate commercial traffic will continue to move through the region unhindered.
China's declaration prompted pushback from the United States and other nations in the region, which rejected the zone and continued to route military flights through the area.
Japan has told its commercial airlines not to comply with the new Chinese demands, but the U.S. government has urged American carriers to follow Beijing's instructions.
On Tuesday, Biden assured Japanese leaders that he would press China over the issue.
"We, the United States, are deeply concerned by the attempt to unilaterally change the status quo in the East China Sea," Biden said at a news conference alongside Japanese Prime Minister Shinzo Abe on Tuesday.
"I will be raising these concerns with great specificity directly when I meet with the Chinese leadership," Biden said.
U.S. State Department spokeswoman Jennifer Psaki reiterated this week that Washington does not recognize the air defense zone or China's demand to be notified of plans by jets from other nations to fly into the area.
The Chinese action, Biden said, raises the possibility of "accidents and miscalculation."
In a statement Tuesday, a spokesman for China's Defense Ministry said the zone is one of "safety, not risks, a zone of cooperation, not competition."
Without naming the United States, spokesman Geng Yansheng, warned countries about becoming involved in China's dispute with Japan.
"Other parties concerned should also mind their words and actions, and should not do things to undermine regional stability and bilateral relations," he said. "Other parties should not be incited, or send wrong signals to make a very few countries go further on the wrong track, which will follow the same old disastrous road and undermine regional and world peace."
CNN's Jethro Mullen and Steve Brusk contributed to this report.