* Two B-1B bombers flew from Guam over the South China Sea
* Bombers conducted night-time training with Japanese fighters
* Flight comes ahead of expected meeting between Trump and Xi (Adds ministry comment, previous TOKYO)
TOKYO/BEIJING, July 7 (Reuters) - Two U.S bombers have flown over the disputed South China Sea, the U.S. Air Force said on Friday, asserting the right to treat the region as international territory despite China's claim to virtually all of the waterway.
The flight by the B-1B Lancer bombers from Guam on Thursday came as U.S. President Donald Trump and Chinese President Xi Jinping prepare for a meeting on the sidelines of a G20 summit in Germany.
The two leaders were expected to discuss what China can do to rein in North Korea's missile and nuclear weapon programmes.
North Korea fired an intercontinental ballistic missile on Tuesday that some experts believe has the range to reach Alaska and Hawaii and perhaps the U.S. Pacific Northwest.
While Trump has been seeking China's help to press North Korea, the U.S. military has, nevertheless, been asserting its "freedom of navigation" rights in the South China Sea, at the risk of angering China.
Asked about the flight by the two U.S. bombers, Chinese Foreign Ministry spokesman Geng Shuang said there was no problem with freedom of navigation or overflight for the East and South China Seas.
"But China resolutely opposes individual countries using the banner of freedom of navigation and overflight to flaunt military force and harm China's sovereignty and security," he said.
China's Defence Ministry, in a short statement sent to Reuters, said China always maintained its vigilance and "effectively monitors relevant countries' military activities next to China".
"The Chinese military will resolutely safeguard national sovereignty and security as well as regional peace and stability," it added, without elaborating.
The United States has criticised China's build-up of military facilities on South China Sea reefs and tiny islands it has constructed, concerned that they could be used to extend its strategic reach.
Brunei, Malaysia, the Philippines, Vietnam and Taiwan also have claims in the sea, through which about $5 trillion in ship-borne trade passes each year.
The two Lancers that made the flight had earlier trained with Japanese jet fighters in the neighbouring East China Sea, the first time the two forces had conducted joint night-time drills.
Two U.S. Lancers flew from Guam over the South China Sea last month, while a U.S. warship carried out a manoeuvring drill within 12 nautical miles of one of China's artificial islands in the waterway in late May. (Reporting by Tim Kelly in TOKYO; Additional reporting by Ben Blanchard in BEIJING; Editing by Paul Tait, Robert Birsel)