Vietnam has captured at least 4,000 refugees trying to flee the country in boats in recent weeks but is unable to cut off the escapes entirely, a Vietnamese official told American congressmen yesterday.

"We think the smaller the number of people leaving the country the better," Deputy Foreign Minister Hoang Bich Son said in an unusual interview in which he submitted to tough questioning by a congressional delegation visiting her to discuss refugees.

He said, however, that Vietnam's "resources are limited" and that some refugees will continue to leave the country.

"The practical circumstances are that there are some people who you cannot keep back," he said.

It is believed to be the first time a Hanoi official has given any indication of the success the government is having in cutting the flow of refugees as it promised to do in mid-July at the U.N.-sponsored Geneva conference on the refugee problem.

The number of escapes has dropped remarkably since that conference and the actual decision to stop the "boat people" from leaving may have been made before the conference.

The Foreign Ministry official dismissed the refugees as people who had led "comfortable" lives in southern Vietnam before the civil war ended but who could not prosper after the American withdrawal in 1975.

Since the Geneva conference, Vietnam has stepped up patrols and increased the number of harbor guards. Son several times emphasized that Vietnam would like to stop all of them from going but lacked the facilities to do so.

The rare question-and-answer session followed soon after the arrival here yesterday of the American delegation headed by Rep. Benjamin Rosenthal (D-N.Y.).

The group received a cool, low-level reception at Hanoi's airport, possibly because the government was angered by one member, Rep. Robert Drinan (D-Mass.), who had accused Vietnam of running concentration camps for dissenters.

At first, Hanoi officials indicated they would retaliate by refusing to receive the delegation but later relented. In a message reinstating the invitation, the government made a wry point by saying that Drinan would be welcomed along with the other eight congressmen.

No high-level officials were present at the airport for the terse welcome, and Son, whom the congressmen interviewed later, is not regarded as a high-ranking official despite his title.

Today the Americans met with Acting Foreign Minister Nguyen Co Thach. The discussions were closed to the reporters who were accompanying the U.S. delegation. At a news conference later, however, Thach said that Vietnam was trying to stamp out the bribery of officials by persons seeking to flee. He also indicated that Hanoi might permit U.S. planes to come here to pick up refugees.

The House group left later today for Manila en route back to Washington.

The delegation flew here from Malaysia after paying a visit to a camp on Bidong Island where about 34,000 refugees are living and hoping for permanent resettlement in other countries.

As their plane approached Hanoi, the House members could look down on fields pockmarked by bombs dropped by U.S. planes during the war. The skeletons of many buildings shattered by those bombs were visible as they rode into the city.

Son and Rosenthal sat side by side in a reception room of the government guest house while members of the delegation peppered the official with several pointed questions.

Rep. John Paul Hammerschmidt (R-Ark.) asked him if the report is true that more than 2 million people still want to leave Vietnam.

"We have no way to confirm that," replied Son, through an interpreter. "We hope the smaller the number of people leaving the country the better." Then he said that the figure of 2 million refugees to come "has no basis."

He said that those who want to leave should be allowed to go but "in a legal manner." He insisted that there is no truth in the common allegation that Vietnam is forcibly expelling unwanted citizens. "There is no government that is more humane than this government," Son said.

Under questioning, Son also defended the so-called new economic zones, where city dwellers have been dispatched to try to begin new lives as farmers. Many refugees have said they fled Vietnam rather than face a life in a zone where they believe they will starve.