KOMATIPOORT, South Africa--President Samora Machel of Mozambique, who led his Marxist nation through 11 crisis-ridden years of independence, was killed when his plane crashed into a rain-swept South African hillside.

Marcelino dos Santos, the number-two man in Mozambique's ruling Frelimo party, said in a nationwide radio address that Machel, 53, and other senior officials died in the crash of the presidential plane Sunday night just across the border in South Africa "in circumstances not yet clarified."

South Africa said all but 10 of the 39 people aboard were killed.

The first official confirmation by Machel's government came more than 12 hours after dos Santos said in his first announcement on the radio that the president's jet had not returned on schedule from Zambia. The radio played solemn music throughout the day.

He said last night that a 60-day period of national mourning had been declared.

The Soviet-built plane crashed 200 yards inside South Africa's frontier with Mozambique about 30 miles south of this border town, said the South African foreign minister, R.F. Botha.

South Africa's Bureau for Information said the Soviet pilot and nine other people survived the crash. Among the senior officials killed was Transport Minister Luis Alcantara Santos, it said.

Machel's body was brought to Komatipoort by truck and flown to Maputo, the bureau said at a temporary office it established here.

Earlier reports put the death toll at 27, but two more bodies were found in the wreckage of the twin-engined Tupolev 134A jet, the bureau said.

In Lisbon, the Portuguese news agency ANOP quoted official sources and aviation experts it did not identify as saying the crash was caused by human error. It did not elaborate.

Machel's death left Mozambique without a leader at a time of worsening conditions in white-ruled South Africa.

Prime Minister Mario Machungo and dos Santos will rule the nation until a successor is chosen after Machel's funeral, expected sometime next week.

Both are considered candidates along with Joaquim Chissano, who was prime minister in a transitional government before independence from Portugal in 1975 and has been foreign minister since.

It was in Komatipoort that Machel and South African President P.W. Botha signed the 1984 Treaty of Nkomati, which was hailed as the start of an era of friendly relations. After signing it, Machel also made overtures to the West for investment and met with President Reagan in Washington.

State Department spokesman Charles E. Redman said the news of Machel's death caused "profound regret."

Secretary of State George P. Shultz had met Machel "and had great respect for him," Redman said, adding that "we extend to the government and the people of Mozambique the deepest condolences of the American people."

There was no indication of foul play in the crash, but the United Democratic Front, South Africa's largest anti-apartheid coalition, said South Africa's recent verbal attacks and diplomatic measures against Mozambique "give us reasonable grounds to suspect South African involvement in this plane crash."

Alfred Nzo, general secretary of the African National Congress guerrilla group, said in Denmark that the ANC had similar suspicions.

Top South African officials would not speculate on the cause of the crash. They said South Africa had proposed a joint investigation with Mozambique and foreign aviation experts should take part.