Defense chiefs of 13 NATO nations today endorsed a rapid increase in the number of anti-tank weapons and artillery shells to be stockpiled in case of attack.

However, suspicions over the future of two new U.S. weapons - the cruise missile and the neutron warhead - are the dominating long-range issues on the minds of many delegates at the opening today of this semi-annual two-day meeting of the North Atlantic Treaty Organization.

Meeting behind closed doors, the delegates here reportedly heard a report on new efforts to bolster NATO's immediate war-fighting efforts that was described by U.S. officials as "reasonably impressive."

Sources said that the alliance had agreed to beef up its inventory of anti-tank weapons - mostly missiles and rockets - by about 45,000 by the end of 1978. This figure represents about a one-third increase in the weapons NATO has to confront a Soviet-led Warsaw Pact tank force estimated at about 18,000 vehicles facing NATO's main flanks.

Sizeable increases in 105mm. and 155 mm. artillery shells were also endorsed.

These immediate efforts plus a few others are part of a pledge made by the NATO ministers at a London summit meeting last May to move on several levels to improve NATO's combat readiness.

Today, U.S. Secretary of Defense Harold Brown and West German Defense Minister Georg Leber reportedly expressed the view that progress was being made in these short-term efforts but both suggested that the pace of badly needed NATO improvement was still lagging in several countries.

Although no names were mentioned, it is clear that at least Turkey, the Netherlands, Portugal and Britain are having either economic or political problems in trying to meet the longer term goals and the pledged 3 per cent real increase in defense spending beginning in 1978.

No major decisions are to be made at this meeting. Rather, the idea is to assess how well the London pledges are progressing and to bring pressure for greater efforts. The aim is to be able to show progress to help convince a skeptical U.S. Congress that Europe is trying harder to defend itself.

Neither the cruise missile nor neutron warhead was discussed here formally today but both are expected to come before the ministers in formal sessions Wednesday.

Authoritative American and European sources report there remains considerable suspicion among the Europeans - especially the West Germans and British - that the United States, in negotiations with the Soviets, will bargain away the European interest in the cruise missile.

European interest in this accurate, relatively inexpensive missile is vast. The West Germans believe it could hit East European targets that would be too expensive to attack using $20 million jets and they see it as a counter to the buildup of intermediate range Soviet SS-20 mobile missiles.

The British see it as a potential successor to their expensive and aging fleet of Polaris-firing missile submarines.