Invading Vietnamese forces pushed deeper today into Cambodia, according to intelligence sources here.

The Vietnamese, who had halted after pushing several miles into Cambodia's eastern frontier last month. They were said to have captured the Mekong River town of Neak Luong in their latest drive. Neak Luong is about 35 miles southeast of Phnom Penh.

[U.S. officials said in Washington that they had no indication that the Vietnamese had moved farther westward into Cambodia from their original positions iiside the border.]

Thai military communications monitors said they are convinced that the Vietnamese commanders are under orders not to cross the Mekong River and move closer to Phnom Penh.

Information about the conflict between the two Communist states is being gathered chiefly through photographs taken by U.S. satellites and reconnaisance airplanes and intercepted radio communications. The Americans have been sharing the material collected by aircraft and statellites with Thailland's supreme military command.

Radio monitors said some Cambodian units have apparently fallen out of contact with higher headquarters. One source said the Vietnamese had bypassed the provincial capital of Svey Rieng in the Cambodian area known as the Parrot's Beak, leaving isolated pockets of Cambodian troops in their wake.