China is gearing up for military exercises in the seas around Taiwan following top US politician Nancy Pelosi's trip to the island.
Ms Pelosi left on Wednesday after a brief but controversial visit to Taiwan, which China regards as a breakaway province.
In response, China announced five days of "necessary and just" military drills, which will begin on Thursday.
Taiwan said 27 Chinese warplanes had already entered its air defence zone.
On Wednesday, Taiwan's defence ministry said it had scrambled jets to warn them off.
The ministry later added that unidentified aircraft, probably drones, had flown over the outlying Kinmen islands, Reuters reported. It said the military fired flares to drive away the aircraft and stayed on alert.
China has said the exercises will take place in some of the world's busiest waterways and will include "long-range live ammunition shooting".
Taiwan has asked ships to find alternative routes to avoid the drills and is negotiating with neighbouring Japan and the Philippines to find alternative aviation routes.
President Tsai Ing-wen said the country was facing "deliberately heightened military threats".
In an effort to calm matters, the foreign ministers of the G7 nations - Canada, France, Germany, Italy, Japan, the UK and US - released a joint statement saying China's escalation risked destabilising the region.
"There is no justification to use a visit as pretext for aggressive military activity in the Taiwan Strait. It is normal and routine for legislators from our countries to travel internationally," the statement said.
Ms Pelosi, the most senior US politician to visit Taiwan in 25 years, made the trip as part of a wider Asian tour. China had warned her not to travel to the island.
Accusing the US of "violating China's sovereignty under the guise of so-called democracy", Chinese Foreign Minister Wang Yi said: "Those who play with fire will not come to a good end and those who offend China will be punished."
وفي بيان بعد الزيارة ، قالت بيلوسي إن الصين لا يمكنها "منع قادة العالم أو أي شخص من السفر إلى تايوان لاحترام ديمقراطيتها المزدهرة ، ولإبراز نجاحاتها العديدة ولتأكيد التزامنا باستمرار التعاون".
بعد مغادرة تايوان ، سافرت بيلوسي إلى كوريا الجنوبية ، حيث من المقرر أن تلتقي برئيس الجمعية الوطنية ، كيم جين بيو.
ولم تتم الموافقة على زيارة العضو الديموقراطي الأمريكي البارز من قبل زميلها في الحزب ، الرئيس جو بايدن ، الذي قال إن الجيش الأمريكي يشعر بأنها "ليست فكرة جيدة في الوقت الحالي" وسط توترات متصاعدة بين البلدين.
الولايات المتحدة تمشي على حبل مشدود دبلوماسي مع سياستها المتعلقة بتايوان. من ناحية أخرى ، تلتزم بسياسة "الصين الواحدة" ، التي تعترف بحكومة صينية واحدة فقط ، وتعطيها علاقات رسمية مع بكين وليس تايوان.
من ناحية أخرى ، تحتفظ بعلاقة "قوية غير رسمية" مع الجزيرة ، والتي تشمل بيع أسلحة لتايوان للدفاع عن نفسها.