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Chinese foreign ministry spokesman Lin Jian said that their "activities in relevant waters and air space are consistent with international law and international practices", and that the two countries were communicating through existing channels.

European fighter jets and warships would also monitor Ukraine's air space and shipping lanes.

"Why would they fly a mission at nine o'clock at night through really busy air space... as opposed to flying that mission at one o'clock in the morning, when there's very little traffic"? he asked.

Located in Virginia at the heart of the capital area, the airport has become exceptionally popular with travellers, but it faces numerous logistical challenges, such as congestion in the air space and shorter runways.

However, there is a lot of restricted air space above the District of Columbia, home to the White House and US Capitol, just across the Potomac River from National Airport.