The development came on the eve of Indonesian President Susilo Bambang Yudhoyono's visit to the disputed area, a military commander said.

"The president will go to the border, therefore we are preparing everything for his security," Military Chief General Endriartono Sutarto said on Sunday when asked about the deployment of the aircraft.

Yudhoyono will visit Sebatik island and the towns of Tarakan and Nunukan during his visit to East Kalimantan, presidential spokesman Andi Mallarangeng told AFP.

Sebatik is off the land border between East Kalimantan province and Malaysia's state of Sabah. The island, just west of the disputed maritime area in the Sulawesi Sea, is split between the two countries.

Tension has risen in the area between Indonesia and Malaysia, following their overlapping boundary claims in the Sulawesi Sea.

Border tensions

"In Sebatik, the president will of course seek direct reports on the situation in the border areas," Mallarangeng said, adding that Yudhoyono would leave for the area on Monday.

Jakarta has protested a move by Malaysia's state oil company Petronas to grant energy giant Shell a concession in the area on 16 February saying that it encroached into Indonesian maritime territory.

Yudhoyono will visit migrants
who have fled Malaysia

Kuala Lumpur has maintained the disputed area lies within its maritime borders.

Indonesia has since sent three navy ships to patrol and guard the disputed area.

"Four more warships are expected to arrive in the same area between Sunday and Monday morning," Indonesian eastern fleet spokesman Colonel Marsetio said, according to the state Antara news agency.

Joint patrols

Warships from both countries came into close contact in the area on Saturday but a standoff was defused when they agreed to conduct joint patrols of the disputed maritime border.

Foreign Minister Hasan Wirayuda said Malaysian officials were ready to discuss the overlapping claims, but added that he was worried the row could "influence relations not only between governments but also between the people".

"All sides should be careful," he said.

Oil operations

Indonesia and Malaysia previously disputed the ownership of two small islands east of Sebatik. But in December 2002, the International Court of Justice ruled that the islands - Ligitan and Sipadan off Borneo island - belong to Malaysia.

Yudhoyono is expected to visit oil operations in Tarakan and temporary holding centres for returning Indonesian migrant workers who fled Malaysia as a government amnesty for illegal migrant workers expired last week.

Thousands of the migrants are sheltering in Nunukan, mostly awaiting proper passports and visas to return to work in Malaysia.