President Carter has dispatched a Marine Corps task force to the Arabian Sea in what administration officials say is a symbol of U.S. determination to back up the president's pledge to defend the Persian Gulf area against any Soviet move to control the region.

The four-ship Marine amphibious unit is carrying 1,800 Marines, plus troop-carrying, helicopters, M60 tanks, 150mm Howitzers and TOW and Dragon antitank missiles.

The amphibious ships, including one helicopter carrier picked up the Marines in Hawaii and arrived yesterday at the U.S. naval base at Subic Bay in the Philippines. The Marines will carry out amphibious landing exercises there for the next two weeks and are to arrive in the Arabian Sea, just South of the Persian Gulf, in mid-March.

Administration officials believe it is the first time in recent memory that Marines have been deployed into the Indian Ocean-Arabian Sea area.

It also marks the first time in the recent crisis that U.S. ground troops have been brought to the region. Thus far, the United States has sent only naval power, in the form of two aircraft carrier task forces, into the Indian Ocean.

These officials said that the order to send the Marines is related only to the Soviet invasion of Afghanistan, and has nothing to do with the holding of American hostages in Tehran by Iranian militants.

The officials emphasized this point because of signs that a solution may soon be found in the Iranian crisis. Thus, the administration does not want to appear to be taking provocative military actions armed of Tehran at this stage.

At a news conference at 8 tonight, President Carter is expected to say whether he sees any hope for an end to the hostage situation soon. Sources said the administration has been studying an Iranian proposed for dealing with the 102-day crisis and that they expect the president to give his response to the plan tonight. (Details on Page A10).

The decision to send Marines to the Arabian Sea area was made two weeks ago, according to adminstration officials, who asked not to be identified, and is not related to reports of recent Soviet troop movements in regions of the Soviet Union just north of Iran.

The Marines will join a task force of 20 Navy ships in the Arabian Sea area, including the aircraft carriers Nimitz and Coral Sea.

The decision to sent the Marines appears to fix into a U.S. military strategy that calls for getting at least a small number of U.S. ground troops into the region before the Soviets could put troops on the ground.

What happens to the Marines once they get to the region is unclear. Keeping them aboard ship for long periods in a warm climate could cause problems that Marine leaders have expressed concern about.

The United States has been negotiating for temporary use of facilities in the region and the new task force could provide the first use of such facilities.

Earlier, official sources reported that the United States is close to agreements for expanded American use of naval and air facilities of Oman, Kenya and Somalia following the return yesterday of a team that visited those countries.

The team, headed by Reginald Bartholomew, director of politico-military affairs for the State Department, is the third such special mission in two months to visit the three countries to investigate greater U.S. military use of their facilities.

All these countries had made clear in the earlier discussious that they were willing in principle, to grant the U.S. request. The latest Bartholomew mission brought the negotiations much nearer to completion, officials said.

U.S. drive to improve its access to established facilities in the area is part of the Carter administration's planned military buildup in the Indian Ocean on the perimeter of the strategic and oil-rich Persian Gulf The decision to seek the use of facilities in Oman, Kenya and Somalia was made before the Soviet invasion of Afghanistan, but has been given additional impetus by that event and the announcement of U.S. determination to protect the Persian Gulf against outside force.

In return for expanded and more regular use of their facilities -- and in stone cases, pre-positioning of U.S. military supplies and equipment for emergency use -- the three countries are expected to receive U.S. assistance, largely in the form of credit sales of military gear. Details and the sums involved in this and other aid have yet to be agreed on, officials said.

Several hundred U.S. personnel would be stationed permanently at the facilities to support American military operations. More than 100 Americans are likely to be stationed at Oman and at Somalia, with a smaller number assigned to facilitate U.S. support operations in Kenya, according to sources.

Of the three sites involved, Oman is considered to be the most important geographically, because of its strategic location overlooking the Straits of Hormuz at the mouth of the Persian Gulf. The United States is expected to operate from Masira Island on the Arabian Sea, a former British base, and from the report of Muscat.

Oman's ruler, Sultan Qaboos Bin Said, has long sought a U.S. connection to replace the British, who withdrew most of their forces from the area a decade ago.

Somalia, on the Horn of Africa along the continent's east coast, is the most controversial site. The United States has edged toward a military relationship with Somalia twice the past three years, but pulled back each time because of that country's military attacks into disputed territory of its Soviet-backed neighbor, Ethiopia. Some officials fear that a military connection with Somalia could embroil the United States in a comflict in the Africa.

Kenya offers the United States use of its port of Mombasa and possibly of a port and airfields which may be inproved in the northern part of that country. Kenyan President Daniel Arap Moi is expected here before the end of this month to confer with President Carter.