Mozambican President Samora Machel attended a surprise summit meeting here today with the leaders of Zimbabwe and Tanzania amid reports the latter two countries are considering providing more military backing to Mozambique.

A noncommittal statement was issued after the four-hour session between Machel, Zimbabwean Prime Minister Robert Mugabe and Tanzanian President Julius Nyerere. Western diplomats here said they believed the three leaders discussed military assistance for Machel in his struggle against insurgents of the Mozambique National Resistance movement.

The summit, held without advance notice, underscored the deteriorating situation in Mozambique, where Machel's Marxist government has been locked in a five-year struggle with the guerrilla organization. The MNR, backed until last year by the white-minority government of South Africa, has forces operating throughout Mozambique..

The rebels recently launched what an MNR spokesman in Lisbon, Portugal, described as a "final offensive" against the capital, Maputo. Although most analysts discount the movement's ability to capture major population centers, Machel last month told a rally in Maputo that the rebels were keeping Mozambique in a state of "partial asphyxiation" that might force him to close schools and factories and send workers to fight because "the survival of the nation was at stake."

Any further commitment of Zimbabwean or Tanzanian troops would be widely interpreted here as a blow to American diplomacy, which has sought to lessen outside military involvement throughout the region and to encourage warmer ties between South Africa and its black neighbors. Mugabe and Nyerere have been Africa's harshest critics of the Reagan administration's regional policy and have attacked a nonaggression accord between Mozambique and South Africa, athough they have refrained from publicly criticizing Machel.

Mozambique signed the pact hoping South Africa would cease support for the MNR, but the movement has continued to flourish..

Machel has also pursued a policy of closer ties to the West in a search for foreign investment for his besieged economy and for support against the rebels. The Reagan administration has supplied $8 million in economic assistance, but an administration proposal to grant a token $1.1 million in military aid was shelved earlier this year because of opposition from conservative Senate Republicans..

Zimbabwe already deploys about 4,000 troops inside Mozambique to protect a railway, road and oil pipeline leading to Zimbabwe from the Mozambican port of Beira.

Tanzania has an undisclosed number of military advisers training and assisting Mozambican troops.

One plan discussed unofficially here would divide Mozambique into three security zones.