Black nationalist guerrillas said yesterday that they were fighting Rhodesian government forces in one of the fiercest battles in six years but the Rhodesian government denied that any major battle was under way.

Guerrillas of the Patriotic Front said in Maputo, Mozambique, that 600 of their forces had entered Rhodesia from Mozambique and penetrated 40 miles into Rhodesia along the south-east border.

They said the battle was one of the biggest since black insurgents opened their campaign to topple Rhodesian Prime Minister Ian Smith's white minority government six years ago.

The guerillas, according to the spokesmen in Maputo, are also distributing leaflets urging Rhodesian blacks to reject the internal settlement that Smith signed with three moderate black leaders March 3.

Following this agreement, there were predictions among observers in Rhodesia that the most serious fighting in years could break out, with guerrilla factions based outside the country stepping up efforts to topple the Smith government to protect their own chances of ruling.

The Rhodesia Herald newspaper, which operates under government censorship of military news, reported yesterday that security forces had routed a 100-strong group of guerrillas believed to belong to the Zimbabwe African National Liberation Army of Robert Mugabe, a coleader of the Patriotic Front.

Reports reaching London from Salisbury said Rhodesian security forces had killed 59 guerrillas and "neutralized" the operation.

Sources in London quoted by Agence France-Presse said the British Foreign Office regards the guerrilla offensive as one of the most important military operations in the region in the last five years.

Rhodesian military headquarters in Salisbury, however, described the guerrilla reports as exaggerated.

"The facts are that a security forces patrol surprised a number of terrorists in a base camp south of Umtali" near the Mozambique border, a military spokesman said. "The number of terrorists surprised is no greater than other groups contacted and destroyed in past actions in this operational area. Routine follow-up operations are in progress."

Meanwhile, the Standard Bank of Rhodesia, in its regular economic buletin released yesterday in Salisbury, said the Rhodesian economy is in its last gasp and it said that only international recognition of a political settlement coupled with an end to the guerilla war can put new life into it.