Americans continue to register disapproval of President Reagan's policies toward Central America despite his sustained drive for public support, according to a Washington Post-ABC News opinion poll.

Many citizens have yet to focus on details of the problems in the region and are unable to state which side the United States is supporting in El Salvador or Nicaragua.

Many agree with Reagan's contention that strife in Central America poses a threat to the security of the United States.

But whether they are informed or not, and whether they see eventual peril or not, the public tends to give Reagan unfavorable ratings for his policies toward Central America and expresses fear of growing entanglement there.

Among the key findings:

* Four in 10 citizens see the United States becoming involved in a new Vietnam to the south, despite the president's repeated statements that events in El Salvador and Nicaragua bear no resemblance to Vietnam.

* Fewer than half those interviewed said they believe the Reagan administration is being truthful when it says it has no intention of sending American soldiers to fight in El Salvador.

* Only 21 percent said they feel that Reagan's handling of the situation in Central America will lead to solving problems in the region, while 27 percent said they feel his approach will exacerbate the problems. Half the public has no view on that question.

* By 54 to 29 percent, citizens said they feel that Reagan is leading the United States more toward getting into war in Central America rather than toward staying out of war there.

Fewer than half those surveyed knew that the United States is backing the government of El Salvador in its war with rebels; only three in 10 knew the United States is opposing the Sandinista government in Nicaragua.

* Six of every 10 people interviewed said poverty and oppression in Central America are more to blame for the unrest there than is subversion from Cuba, Nicaragua and the Soviet Union.

* The public is about evenly split on the desirability of the large-scale U.S. land and sea military maneuvers scheduled for Central America. Forty-four percent said they approve of the maneuvers; 46 percent said they oppose them. However, only 19 percent said they accept the view, expressed by Reagan, that the maneuvers are similar to those conducted by the United States in the past, and 67 percent said they regard them instead as a show of American might aimed at hostile powers in the area.

Sixty-two percent said they disapprove of any U.S. involvement in overthrowing the government in Nicaragua; only 20 percent said they approve.

The new poll, conducted July 28 to Aug. 1, is the third on Central America done by The Post and ABC News since March, 1982. For the most part it shows hardly any change in attitudes over that 18-month period. Overall, 48 percent said they disapprove of Reagan's handling of the problems in Central America, and 33 percent said they approve.

On a number of occasions, the president has blamed the news media, at least in part, for the lack of public approval of his policies toward El Salvador and Nicaragua. According to the poll, however, most citizens approve of the news media's coverage of Central America, and many people trust the news accounts of events in El Salvador more than they trust Reagan's statements.

At a news conference last week, Reagan, responding to a letter from a 13-year-old, said that he wished he could stamp his feet and shout at the press to be quiet, sit down and listen to what he is saying. He also said the media, through a "constant drumbeat," had raised suspicions that he had an "ulterior purpose" in conducting the military maneuvers.

One question in the poll noted that Reagan has been critical of the press, and asked people how they rate news coverage of Central America. Ten percent said the coverage was excellent, 51 percent said it was good, 23 percent said it was not so good, 9 percent said it was poor and 7 percent expressed no opinion.

A second question noted that "sometimes what Reagan says about his policies in Central America and what the news media report seem to be in conflict" and asked people whether they tend to believe Reagan or the news media more in such instances. Forty-nine percent said they believe the press more, and 37 percent said they believe Reagan more.

Reagan also said last week that he gets support for his Central American policy from citizens who "have been informed and understand it." The poll shows that on some specifics Reagan does get more support from those with a higher degree of knowledge--but not much more. And in other instances, he gets less support from them.

Some of the sharpest differences in outlook are partisan ones, with Republicans expressing confidence and trust in the president and Democrats sharp distrust. On the question of whether the United States is headed toward another Vietnam, Democrats said 'yes' by a 50-to-44 margin; Republicans said 'no' by 72 to 23.

On the question of whether the Reagan administration is telling the truth in saying it has no intention of sending troops to fight in El Salvador, Republicans said 'yes' by 68 to 25 percent, but Democrats said 'no' by 53 to 36 percent.

By 66 to 17, Democrats said that Reagan is leading the nation more toward war in Central America. By 50 to 31, Republicans said he is leading us more away from war.