China however placed the blame on the US plane.
The Chinese plane apparently collided with a US Navy EP-3 surveillance aircraft which was forced to make an emergency landing on Chinese soil.
None of the American crew were hurt, but rescue parties are searching for the Chinese pilot whose whereabouts are not known.
The US Pacific Command said their aircraft issued a mayday signal and was forced to land at an airbase on Hainan Island.
They said the four-engine propeller plane was on a routine mission in international airspace at the time of the collision.
US crew safe
Pacific Command spokesman Colonel John Bratton said the collision appeared to be "an accident".
"The planes actually bumped into each other," another spokesman, Lieutenant Colonel Dewey Ford, said.
China however placed the blame on the US plane.
''A Chinese aircraft was conducting normal flight operations 10km (6 miles) south of Hainan Island when a US plane suddenly veered towards it,'' Chinese state television quoted the Foreign Ministry as saying.
The ministry said it had lodged a protest with US authorities and reserved the right to seek damages.
There were no reports of injuries among the 24 US crew members - one member of the Air Force, one marine, and 22 navy personnel - on board the aircraft.
The US ambassador in Beijing, Joseph Prueher, said American diplomats were on their way to Hainan to see the crew members.
He said he had talked several times with US Secretary of State Colin Powell and is also reported to have had an initial meeting with the Chinese vice foreign minister.
The BBC's correspondent in Washington, Richard Lister, says the US is moving swiftly to defuse what could become a major diplomatic row.
Experts say run-ins between Chinese and US aircraft are quite common along the Chinese coast, although it was the first time an aircraft had made an emergency landing.
"It's very regular for the American navy to have their planes intruding into Chinese airspace," Yan Xuetong, an international studies expert at Beijing's Tsinghua University said.
"The Chinese then send up fighters and chase them out."
Uneasy relations
It comes at a time when Sino-American relations are under increasing strain.
US plans to develop a national missile shield - the so-called "son of star wars" - are a particularly contentious issue.
US Navy Commander Rex Totty said: "We expect that the government will respect the integrity of the aircraft and the well-being and safety of the crew in accordance with international practices, expedite any necessary repairs to the aircraft and facilitate the immediate return of the aircraft."
The US plane had taken off from Kadena Airbase in Okinawa, Japan.