US secretary of state Antony Blinken has said that China's military exercises aimed at Taiwan represent a "significant escalation".
At a news conference in the Cambodian capital Phnom Penh, Mr Blinken said: "China has chosen to over-react and use Speaker Pelosi's visit as a pretext to increase provocative military activity in and around the Taiwan Strait."
Mr Blinken also said the US stands in "strong solidarity" with Japan following the "dangerous actions China has taken" – with the exercises including missiles fired into Japan's exclusive economic zone.
Watch: China conducts live-firing drills near Taiwan.
China's military drills were launched following a visit earlier this week by US house speaker Nancy Pelosi to Taiwan that infuriated officials in Beijing.
After Ms Pelosi and other US representatives landed on the island, China announced military exercises were under way in six zones surrounding Taiwan, which Beijing claims as its own territory to be annexed by force if necessary.
Along with its moves to isolate Taiwan diplomatically, China has long threatened military retaliation over moves by the island to solidify its de facto independence with the support of key allies including the US.
Earlier this week, 10 nations from south-east Asia called for calm in the Taiwan Strait and urged against any "provocative action" in the wake of Ms Pelosi's visit.