Gunmen loyal to Muammar Gaddafi have crossed the Libyan border from Algeria and attacked revolutionary forces in a town near the frontier, killing six people, officials said on Sunday.

The cross-border attack on Saturday shows loyalist forces have managed to escape Libya and regroup and collect arms, bolstering fears the North African nation could face a protracted insurgency.

Fighters who took up arms against Gaddafi have seized Tripoli and have gained control of the rest of the country, but they are still battling forces loyal to the ousted regime on several fronts.

Libya officials also announced on Sunday the discovery of a mass grave believed to hold the remains of 1,270 inmates killed by Gadhafi's regime in a 1996 prison massacre.

The site — a desert field scattered with bone fragments — was found outside the walls of Tripoli's Abu Salim prison, where the victims were killed on June 26, 1996, after protesting conditions at the facility.

A demonstration by women demanding justice for the victims of that prison massacre was one of the things that touched off the uprising against Gaddafi in February.

A Tripoli military spokesman, Khalid al-Sharif, said authorities found the site after getting information from witnesses and former security guards who had been captured after the capital fell.

Officials will ask for international assistance in excavating and identifying the remains because the Libyans don't have sufficient expertise and equipment to test the DNA, he said.

Libyans are eager for those who committed crimes under the old regime to face justice and have been moving forward with efforts to account for the past even as fighting continues in parts of the country.

Col. Ahmed Bani, a military spokesman for the transitional government, said the attack on Ghadamis occurred on Saturday but revolutionary forces had intelligence that cars filled with weapons had crossed the border a few days earlier. Ghadamis is about 280 miles (450 kilometers) southwest of Tripoli.

He said the loyalist forces were believed to belong to a unit that had been under the command of Gaddafi 's son Khamis, who was reportedly killed in fighting before the revolutionary forces seized Tripoli.

Bani said revolutionary forces had repelled the attack but the assailants escaped back across the border.

An official from Ghadamis, Ali al-Mana, however, said fighting was ongoing. He told The Associated Press that six people had been killed and 63 wounded.

"We are sending a plane from Tripoli to evacuate the wounded," said al-Mana, who is the Ghadamis representative on the National Transitional Council, which is acting as the country's government. Al-Mana said Ghadamis has a small runway for the plane to land.

Gaddafi's wife and three of his children, including his daughter Aisha, fled to Algeria through Ghadamis after Tripoli's fall late last month.

The whereabouts of the fugitive leader remain unknown and he continues to try to rally supporters. That has raised concern that he could stoke violence as fighting continues between revolutionary forces in his hometown of Sirte and two other strongholds.

Aisha Gaddafi , who played a key role in her father's inner circle, said in an audio recording broadcast Friday that her father is in high spirits and fighting alongside his supporters.

In other developments on Sunday, Gaddafi 's hometown of Sirte came under an extremely heavy NATO bombardment as opposing forces laid siege to the city.

An AP reporter in Sirte said there were dozens of airstrikes throughout the day in and around the city, possibly softening up targets for a new push by revolutionary forces in the coming days.

Sirte, 250 miles (400 kilometers) southeast of Tripoli on the Mediterranean coast, is the Libyan city most associated with Gaddafi.

An offensive by anti-Gaddafi forces on Saturday failed to dislodge die-hard loyalists of the fugitive leader.

Anti-Gaddafi fighters set up new checkpoints and posted snipers in strategic areas on the outskirts of Sirte.

But they did not launch an assault after facing fierce resistance on Saturday that left seven of their comrades dead and more than 150 wounded.

Osama Nuttawa al-Swehli, a revolutionary logistics officer, said the goal on Sunday was to squeeze the city and prevent any former regime figures believed to be holed up inside from escaping.

Al-Swehli said he has heard Gaddafi's son Muatassim communicating by radio with loyalist forces inside Sirte.

"We have to make sure that no supplies get in and that none of their assets escape," he said.