SINGAPORE (Reuters) - A rare direct flight from Pyongyang landed in Singapore on Saturday ahead of the expected arrival of North Korea’s leader Kim Jong Un for a summit with U.S. President Donald Trump to discuss ending the North’s isolation and its nuclear program.

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The arrival of the Air China Airbus 330 flight at Changi airport raised speculation that a delegation of North Korean officials joined an advance team headed by Kim’s close aide, Kim Chang Son, who was in Singapore to prepare for the talks.

North Korean leader Kim is expected to arrive at Changi airport on Sunday, a source involved in the planning of the trip told Reuters on Friday.

There was heightened security at the St. Regis hotel in central Singapore, one of the hotels where the leaders were expected to stay.

Plainclothes police and the Singapore Army’s Explosives and Ordnance Disposal unit were seen working to prepare for a high-security event.

The leaders meet on Tuesday on the resort island of Sentosa and talks are expected to center on ending the North’s nuclear weapons and missiles programs in return for diplomatic and economic incentives.

Singapore Foreign Minister Vivian Balakrishnan wrapped up a visit to Washington and Pyongyang on Saturday and told reporters in Beijing both sides expressed thanks for Singapore’s work to prepare for the summit on a short notice.

“I’m glad to say that as of now, it’s all systems go. Both the American as well as the North Koreans are pleased with the arrangements, we’re all set to go and things will start happening within the next 24 hours,” he said.

The diplomatic initiatives leading to the summit started in late April when Kim met South Korean President Moon Jae-in and the leaders of the rival states pledged to end hostility and establish peace on the Korean peninsula.

On May 10, Trump announced he would hold a summit with Kim in Singapore and has since indicated he would try to reach an agreement to formally end the 1950-53 Korean War.