Taiwan's defence minister said on Tuesday that they have a "grasp" of the situation after pictures appeared online of a Chinese nuclear submarine surfacing in the sensitive Taiwan Strait near Taiwanese fishermen.

The narrow strait that separates Taiwan from China is a frequent source of tension. Taiwan reports Chinese warplanes and warships operating there on a daily basis, as Beijing seeks to assert its sovereignty claims against the democratically governed island.

Taiwanese media published the pictures of the surfaced craft, which appears to be a nuclear-armed Jin class ballistic missile submarine, taken by a Taiwanese fishing boat in the strait as dawn broke on Tuesday, about 200 km (125 miles) from Taiwan's western coast.

Asked about the submarine, Taiwan Defence Minister Wellington Koo said they have a "grasp" of the intelligence situation, but declined to say how they were monitoring it or give details.

China's defence ministry did not immediately respond to a request for comment.

Nuclear-powered submarines can operate underwater for months at a time, and ballistic-missile boats' secretive mission means they rarely surface.

A security source familiar with the situation told Reuters that the submarine was most likely returning to its home port in Qingdao from the South China Sea. The source said Tuesday's incident might have been because it experienced a malfunction and was forced to surface.

The source spoke on condition of anonymity given the sensitivity of the situation.

Military experts say the strategic waters off Taiwan's southwestern shores, where the largely shallow Taiwan Strait descends in depth, provide submarines a location for an ambush, making it a hot spot for militaries including China, Taiwan and the United States.

Ballistic missile submarines are not designed to attack ships, but to launch ballistic missiles at targets on land.

Taiwan's fleet of P-3C Orion anti-submarine aircraft are based at the Pingtung air base in southern Taiwan, giving easy access to the southern part of the strait.

Taiwan has complained in recent years that China has been using so-called grey zone warfare designed to exhaust a foe without resorting to open combat, such as flying surveillance balloons over the island.

"We must be fully alert to China's continued military harassment and grey zone threats and must always understand China's constant salami-slicing attempts to unilaterally change the status quo," Koo said.

"We must be alert at all times, but not panic nor be apathetic, and calmly deal with the situation in the strait," he added. "We won't be the one provoking, and call on China not to be a troublemaker."

Taiwan detected 20 Chinese military planes and seven vessels around the island in the past 24 hours, Taiwan's defence ministry said in its daily report on Chinese military activities on Tuesday morning. (Reporting by Ben Blanchard and Yimou Lee; Editing by Gerry Doyle)