The State Department, in an unusual public announcement, yesterday attacked "slanderous and baseless" Soviet allegations regarding U.S. interference in Afghanistan and suggested that the allegations are a smokescreen for increased Soviet involvement.

The U.S. statement, volunteered to reporters in late afternoon, is one of a series expressing Washington's concern about growing Russian activity in the thinly populated mountain state on the Soviet Border.

The United States is unusually sensitive to Soviet charges and actions there because of the unsettled conditions of the region following the events in Iran and because of the killing of U.S. Ambassador Adolph Dubs on Feb. 14 in Kabul, the Capital of Afghanistan.

Some administration officials speculated that the new Soviet press attacks on the United States may be part of a campaign by Moscow to justify greater military support for the revolutionary Afghan regime, which came to power in a coup a year ago this month. In recent weeks, the Russians are reported to have stepped up their shipment of military equipment, including large helicopters, as the Afghan regime has encountered increasing opposition from Moslem tribesmen and others.

A Pravda article under an authoritative pseudonym on March 19 drew a strong rebutal from the State Department March 28, along with a waring to the Soviet Union to stay out of Afghanistan's internal Battles.

A Pravda article and a television interview with a high Soviet official, both on March 29, were the point of departure for yesterday's State Department announcement. High-level administration deliberations reportedly led to the annoucement.

According to the State Department, the latest Russian charges were that the United States has supported Afghani insurgents. The television statements, by Leonid Zamyatin, cheif of the information department of the Central Committee of the Soviet Communist Party, reportedly charged that Deputy Secretary of State Warren Christopher had met with Pakistani representatives for the purpose of arranging arms supplies to the insurgents.

The State Department called the charges of U.S. interference in Afghanistan "slanderous and baseless," and said the United States takes a critical view of "false statements by a high Soviet official" impugning senior American officials.

"Such untruths continue to raise questions about the intentions of the U.S.S.R. with respect to interference in the internal affairs of Afghanistan," the State Department said.

On Capitol Hill, Sen. Claiborne Pell (D.R.I.) issued a statement charging the Soviet press with "fabricating stories of United States involvement" in Afghanistan to cover the actions of their country. Aides to Pell said he learned of the Soviet allegations from administration officials.

Pell, a senior member of the Foreign Relations Committee, has taken a special interest in Afghanistan following the killing of Dubs, whose widow works in a Capitol Hill office next door to that of the senator.