HO CHI MINH CITY, March 11 -- A Vietnamese AH-26 patrol plane rumbled Monday over the glittering sea off the Gulf of Thailand, as crew members watched intensly at the sea surface for any possible sign of the missing Malaysian jetliner.

Two groups of Xinhua reporters were allowed to board air planes of Vietnamese air force and witnessed this arduous search for hope, four days after the ill-fated plane mysteriously disappeared in waters between Malaysia and Vietnam last Saturday.

After a 45-minute flight, the AH-26 patrol plane reached the designated search site and lowered its altitude to gain a clearer view of the sea surface. Eager to find somethig that may lead to the whereabouts of the missing aircraft, these reporters looked hard through the portholes at the sea below. Yet, what came into their view was only the serene sea surface dotted here and there with merchant ships and fishing vessels against a clear blue sky.

Durig the flight, two crew members at each side of the plane would poke their head out to watch closely for any suspicous objects on the sea and make entries into the flight log from time to time. Another crew member would keep watch through a porthold at the rear of the aircraft. The whole operation seemed to proceed in a well-ordered way.

At the end of the search that lasted hours hours and a half, Captain Vu Duc Long told reporters that unfortunately no meaningful clues to the missing aircraft was found in today's search that covered an area of some 400 square km though he had flew his plane only 500 meters above the sea surface for a better view.

Boarding a helicopter of the Vietnamese air force, another group of Xinhua reporters joined a two-hour cruise to examine a suspected oil slick. The copter flew at an altitude of 100 meters and a speed of 200 km per hour to serch an area of 90 km long and 70 km wide for suspected oil slicks.

The crew, composed of navy and air force prsonnel, made observations with naked eyes through opened cabin doors. As the good weather offered high visibility, Xinhua reporters sighted several small floating objects other than ships during the flight, all dismissed as irrelevant to the missing plane.

When the helicopter reached the destination, no suspected oil slick was found. An air force officer told reporters the copter had flewn two sorties every day since the Malaysian jetliner disappeared en route from Kuala Lumpur to Beijing.

Do Minh Tuan, deputy commander of Vietnam's air force told a press conference eralier Tuesday that the search efforts by Vietnam on the air and at sea have so far failed to yield any positive results, but the country would continue the search.

Vietnam has sent 10 sorties of aircrcft into this major search operation and will ask China to send two more aircraft as reinforcement, he said.

Asked when the search would come to a close, the air force commander said the operation would continue and probably at a bigger scope, and that no time limit would be set.

The MH 270 flight, operated by a Boeing 777-200 aircraft, left Kuala Lumpur International Airport at 0:41 a.m. Beijing time on Saturday and was expected to land in Beijing at 6:30 a.m. the same day.

Contact with the flight was lost along with its radar signal at 2:40 a.m. Beijing time on Saturday when it was flying over the Ho Chi Minh air traffic control area in Vietnam.