The Carter administration acted yesterday to expedite arms shipments to North Yemen even as neighboring Saudi Arabia notified the United States that it did not require a squadron of F15 fighter planes to help it maintain its security at this time.

The president, in a letter signed just before he left for the Middle East, advised Congress that the shipment of some $400 million worth of planes, tanks and other equipment to North Yemen would commence immediately, without the usual 30-day congressional review.

Under arms sales law, the president can dispense with the review if he finds, as he did in this case, that an emergency exists that vitally affects U.S. interests.

Justification for the American buildup in the area is the two-week-old border war between pro-western North Yemen and pro Soviet South Yemen.

Pentagon officials said yesterday that 800 to 1,000 Soviets, 300 to 500 Cubans and at least 100 East Germans are acting as military advisers and technicians in South Yemen, but there is no evidence that they have been used in combat.

According to the officials, South Yemeni forces penetrated 20 miles across the border and threatened to cut the road between Sana, the capital, and Taizz before being turned back by North Yemen troops Tuesday. The fighting is taking place in rugged mountain terrain inhabited by tribes and clans that have always been far removed from the geopolitics that often swirls around the area.

Administration officials stressed yesterday that stepped-up U.S. military aid for North Yemen has been sought both by that country and by Saudi Arabia for months.

However, several elements of Arab caution on the American arms buildup enterod the picture yesterday.

One was a State Department announcement that Saudi Arabia had declined a U.S. offer to send a squadron of F15s based at Langley Air Force Base in Virginia to shore up Saudi security. The offer was a key part of a hastily devised Carter strategy to demonstrate American determination in the area.

State Department officials that the offer was still open and could be accepted later.

Two large U.S. radar reconnaissance planes, or AWACS, arrived in the Saudi capital of Riyadh yesterday. Pentagon officials declined to give details of their mission other than to say the advanced surveillance planes would have only a "passive" role above Saudi territory. The planes are accompanied by at least three C141 transport aircarft carrying backup crews and equipment.

Meanwhile, State Department officials said they were "baffled" by a statement from North Yemen president Ali Abdullah Saleh accusing both the United States and the Soviet Union of playing a "superpower game" in the Arabian peninsula.

Saleh, in an interview with a Kuwait newspaper, said South Yemen's aim was to transform Yemen into an area of "dangerous superpower conflict."

Yesterday, the State Department instructed the U.S. ambassador to seek a clarification of the remarks, which appeared to lump the Soviet Union and the United States together as potential troublemakers in the area.

Administration sources yesterday described Saleh as a man who "has not been a strong or popular leader." Officials said that Saleh had been pressing the United States for a speedup of arms aid almost since taking power following his predecessor's assassination last June.

The United States has closely coordinated policy with Saudi Arabia and North Yemen since mid-1978. This policy includes a large North Yemen arms package, paid for by Saudi Arabia, to counter Soviet-supported South Yemen.

The recent American moves, officials insisted yesterday, have had the full support of the Saudis.

"We are not forcing this equipment on them, we are responding to their requests," said one official. "The Saudis have paid for the entire show."

Some sources believe, however, that the Saudis are looking for ways to tone down the growing sense of an Iranian-style American arms buildup in the area. Arab foreign ministers have been in close consultation through the Arab League since last weekend. Some Arab spokesmen say they are anxious to keep the option of more American aid open, but do not wish to create a new forum for superpower rivalry.

Pentagon officials, on the other hand, say that Saudi concern over events in Yemen runs deep. Some one million Yemenis now live and work in Saudi Arabia, and instabiity in the Yemeni countries creates tensions among these segments of the Saudi population.

Pentagon officials say that, at present, North Yemen is up against a Soviet, Cuban and East German-trained army equipped with 50 Mig21s, other aircaft and 130-millimeter artillery with ranges far outdistancing anything in North Yemen.

Paradoxically, the officials said, some 100 Soviet military advisers still are in North Yemen under an old Soviet aid program to that country now being phased out.