President Joko Widodo visited remote Indonesian islands on a warship Thursday in an apparent show of force after clashes with Chinese vessels and as fears grow Beijing is seeking to stake a claim in nearby waters.

Widodo led a high-level delegation including the foreign minister and armed forces chief to the Natuna Islands in the South China Sea, arriving at a navy base before being escorted to the warship as fighter jets buzzed overhead and military vessels performed manoeuvres off the coast.

At a meeting of ministers and security force chiefs on the warship, which last week detained a Chinese trawler and its crew in Indonesian waters, the president ordered defences around the Natunas to be stepped up.

"I asked the military and the maritime security agency to better guard the seas," he said. A picture released by the government showed Widodo standing next to a gun turret on deck, flanked by the military chief and ministers.

Before the trip, Security Minister Luhut Panjaitan said it was aimed at sending a "clear message" that Indonesia was "very serious in its effort to protect its sovereignty".

Beijing asserts sovereignty over almost the entire strategically important South China Sea, and regional tensions are mounting due to Chinese island building and ahead of a UN-backed tribunal's ruling on a Philippine challenge to China's claims.

Unlike some of its Southeast Asian neighbours, Indonesia has long maintained it has no maritime disputes with China in the sea and has no overlapping claims to reefs or islets there.

But Beijing's claims overlap Indonesia's exclusive economic zone -- waters where a state has the right to exploit resources -- around the Natunas, and Widodo's visit came after a sharp escalation in maritime clashes between Indonesian vessels and Chinese fishing boats in the area.

- 'Overlapping claims' -

Foreign ministry spokeswoman Hua Chunying Thursday reiterated Beijing's position that China and Indonesia "have no territorial disputes" and that China does not object to Indonesia's sovereignty over the Natuna Islands.

But she added that "China and Indonesia have overlapping claims over maritime rights of some part of the waters in the South China Sea".

The term "overlapping claims" was also used in a Chinese statement earlier this week. Analysts say the language indicates Beijing is taking a tougher stance by openly saying that China and Indonesia have competing maritime claims.

The growing tensions come after a senior US State Department official warned this week that China is using its fishing fleets with armed escorts to bolster maritime claims.

The latest confrontation between Beijing and Jakarta came last week when the Indonesian navy seized a Chinese-flagged fishing vessel and detained its crew for allegedly operating illegally in Natuna waters. It was the third such skirmish between vessels from the two countries in the area this year.

Beijing protested and claimed that one fisherman was injured after Indonesian vessels fired warning shots. Jakarta says none of the crew were hurt.

Widodo's visit to the islands, which are located west of Borneo, was his first as president. The Natuna waters are home to oil and gas deposits as well as fishing grounds.

Confrontations between Indonesian and Chinese vessels around the Natunas have increased since Jakarta launched a crackdown on illegal fishing in 2014.