A top leader of Rhodesia's black nationalist guerrillas warned here yesterday that the "internal" settlement accord apparently about to be concluded in Salisbury will result in increased fighting both inside and outside that already war-torn nation.

"We are going to hit each other hard," said Joshua Nkomo, co-leader of the black nationalist guerrilla alliance, the Patriotic Front. "We intend to finish him up," he said, referring to the leader of Rhodesia's white minority government, Prime Minister Ian Smith.

Nkomo's remarks, and a statement issued yesterday by the Zambian government, seemed to confirm the general impression of western observers here that the guerrillas and the "front-line" states backing them are bracing themselves for a sharp escalation in the war and its probable spread into neighboring countries on a much larger scale than before.

The United States yesterday toned down its criticism of the internal peace agreement. State Department spokesman Kenneth Brown said:

"On the basis of what we know, it represents only a part of a much larger and more comprehensive set of arrangements which must be worked out prior to the beginning of the transition, let alone independence. Until we see a text and have had an opportunity to consider it carefully, we will have no further comment."

Brown had initially criticized the plan Wednesday specifically because the Patriotic Front leaders had not participated in it.

In London, British Foreign Secretary David Owen, under pressure from Conservative politicians to accept the Smith agreement, told parliament that the accord is "a significant step toward majority rule." He said it would be "most unwise to make judgments until we have far greater detail."

In an interview, Nkomo made it clear he has no intention of striking any political deal with Smith and returning to Salisbury to enter the multiracial interim government being established under the settlement plan, Smith reached with black leaders living inside Rhodesia.

"Nonsense. You can forget me. I do not go in for sham agreements," he said, adding that it would be a "curse on anybody" to join a government "completely controlled by racists and fascists."

While asserting that the Salisbury agreement now in the making, "will not work," Nkomo seemed to welcome the new development because, he said "the problem has been simplified . . . Muzorewa and Sithole are now part of the Smith regime. We face one enemy now."

The Bishop Abel-muzorewa and the Rev. Ndabaningi Sithole are two of the three African nationalist leaders who negotiated with Smith. The other is Chief Jeremiah Chirau.

Nkomo predicted that the settlement scheme would bring neither peace nor stablitiy to Rhodesia because, "We are not going to allow it."

The 60-year-old nationalist is president of the Zambia-based Zimbabew National People's Union (ZAPU), one of the two factions making up the Front. It has a Soviet-armed and Cuban-trained guerrilla force of 8,000 to 10,000 men, the majority of whom are just now becoming ready for combat.

The other faction, the Zimbabwe African National Union (ZANA), led by Robert Mugabe, is based on the other side of Rhodesia in Mozambique. It has a slightly larger force armed and backed in part by China. The Smith government says ZANU has so far been fielding most of the guerrillas fighting inside Rhodesia, although Nkomo hotly disputed this assessment, saying, "We are there in full force."

Nkomo said he was confident that the front-line states would continue to back the Patriotic Front even if they increasingly become the battleground as the Rhodesian army steps up its raids on Guerrilla camps in Zambia, Mozambique and Botswana. This is what is generally expected to happen now.

"They have gone through worse things than this," Nkomo said. "They are in this because they believe in the complete indepedence of those living under colonial, fascist rule."

The Zambian government yesterday reaffirmed its support for the Patriotic Front and also renewed its backing of the British-American peace plan as the only basis for a "genuine and permanent independence settlement in Rhodesia." The British-American plan is being negotiated with the Front.

"It would be unthinkable to take the internal settlement seriously," a Foreign Ministry statement said. "Zambia has never been a part of the internal settlement. As far as we are concerned the internal settlement can never end the war in Zimbabwe," the nationalists' name for Rhodesia.

The statement was apparently aimed at dispelling speculation that Zambia, like Nkomo, might be lured now into giving support to the internal plan engineered by Smith, Zambian President Kenneth Kaunda held a secret, day-long meeting last September at his presidential state house with Smith, arousing suspicions that the two leader were discussing some kind of bilateral deal.

The Foreign ministry declaration also indicates that the Zambian President, after withdrawing his support for the British-American proposals in November, is once again backing them. Kaunda had taken issue with the British American objective of holding elections before establishment of a black majority government. Instead, he favored a direct handover of power to the Front.

Kaunda failed, however, to gain the backing of either Tanzanian President Julius Nyerere or Mozambigan President Samora Machel.

There has been considerable debate in Western diplomatic circles and among other foreign observers here over what Zambia will do if confronted by an internal settlement and the ugly prospect of repeated Rhodesian raids into this country.

Zambia is facing a serious economic and financial crisis already, partly as a result of its long commitment to the black nationalist cause, and no immediate relief is in sight. The country is also scheduled to hold both presidential and parliamentary elections later this year.

A strong undercurrent of public opinion here is fed up with the Rhodesian war and resentful of the heavy cost of the Zambian economy of the country's involvement in the conflict.

So far Zambia has not suffered as much as Mozambique from Rhodesian raids on the guerrilla camps along the border with Rhodesia. There are increasing signs, however, that the Rhodesians intend to step up their attacks into this country now that Nkomo shows no signs of returning to Salisbury and the ZAPU guerrilla force poses a much more serious threat than before.

The Rhodesians carried out their biggest known attack on a ZAPU camp Feb. 7, using helicopters to ferry commandos across Lake Kariba and apparently more than 60 miles into Zambia. Reports said anywhere from 50 to 80 guerrillas and Zambians were killed in the raid, including eight Zambian security force members whose Land Rover was blown up by a landmine.

Three Zambian civilians were killed and a member of parliament was wounded last weekend by other landmines placed on roads in the Gwembe Valley, the reported site of the Feb. 7 Rhodesian raid.

THe Zambian government has officially denied that the Rhodesian raid took place, apparently because of its concern about the repercussions of the incident on internal politics during an election year.

It is precisely this Zambian sensitivity to such incidents that has stirred speculation about how much punishment from further Rhodesian raids the government can withstand before slackening in its support for the Partiotic Front.