TAIPEI, March 31 (Reuters) - Nine Chinese aircraft crossed the median line of the Taiwan Strait on Friday carrying out combat readiness patrols, Taiwan's defence ministry said, condemning what it called an "irrational" deployment.

The rise in tension comes as Taiwan President Tsai Ing-wen is in the United States stopping off on her way to Central America.

She is expected to meet U.S. House Speaker Kevin McCarthy in Los Angeles on her way back to Taipei in April, and China this week threatened unspecified retaliation if that meeting were to do ahead.

Taiwan's defence ministry said the nine Chinese aircraft crossed at points in the north, centre and south of the strait's median line, which used to serve as an unofficial buffer between the two sides.

Taiwan's armed forces responded using its own aircraft and ships to monitor the situation using the principle of "not escalating conflicts or causing disputes", the ministry said.

"The communist military's deployment of forces deliberately created tension in the Taiwan Strait, not only undermining peace and stability, but also has a negative impact on regional security and economic development," it said in a statement.

The ministry condemned what it called "such irrational actions".

There was no immediate response from China, which claims Taiwan as its own territory.

China staged war games around Taiwan last August following the visit to Taipei of then U.S. House Speaker Nancy Pelosi, and has continued its military activities near Taiwan since though on a reduced scale.

China has never officially recognised the median line, which a U.S. general devised in 1954 at the height of Cold War hostility, although the People's Liberation Army had until very recently largely respected it.