NATO is planning raids on Ras Jedir, a border post with Tunisia, to open the way to a rebel advance, Libya's government spokesman said on Sunday.
"We have information that NATO is planning intensive bombardments of the Ras Jedir border post to help the armed bands, some of whom are positioned on the Tunisian side, to enter Libyan territory," Mussa Ibrahim told AFP.
He described the North Atlantic alliance's intentions as "very dangerous," pointing out that Ras Jedir was a crossing point for "thousands of Libyans, including women and children."
"By bombing it, NATO will be committing a new massacre," Ibrahim said, warning against an "illegal act."
The Libyan government insisted Zawiyah remained under regime control on Saturday, dismissing rebel claims of having captured the port city near Tripoli as the revolt nears the six-month mark.
Zawiyah is completely under government control, Ibrahim also told reporters.
The rebels earlier told an Arabic satellite channel they had captured the town, as a bus transporting foreign journalists travelling from neighbouring Tunisia was forced to turn back as it neared Zawiyah.
Ibrahim said less than 100 fighters tried to enter the town to join up with about 50 rebels inside Zawiya.
"This is not an advance. This is what you call a skirmish, what you call a suicide mission," he said. "You have to remember we are very powerful. Tens of thousands of volunteers are armed right now."
In a spurt of military activity, rebels fighting east of Tripoli said they strengthened their grip on the town of Tuarga in a bid to create a buffer zone between Moamer Kadhafi's forces and the city of Misrata.
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Two fighters died and 15 were wounded as they closed in on the last sniper and pro-Kadhafi artillery positions in the town, the rebels said. The bodies of at least four Kadhafi loyalists were seen at a field hospital.
Rebels also said they gained ground on Saturday in the government-held oil town of Brega.
"We have taken control of residential area number 2, after number 3 yesterday," out of four zones, a military spokesman, Mohammed Zawiwa, told AFP. "The pipe factory has fallen into our hands today."
Libya earlier criticised UN chief Ban Ki-moon after he acknowledged NATO efforts to avoid civilian casualties, a day after urging restraint over "unacceptably" high deaths.
"The secretary general has consistently called for restraint and caution to avoid civilian casualties. He of course recognises and appreciates NATO's efforts to avoid civilian casualties," a UN spokesman told reporters Friday.
In a statement released by his office on Thursday, Ban sounded the alarm over civilian deaths and called for new efforts to find a political solution between Kadhafi and opposition rebels.
Without specifically naming any side, Ban called on "all parties" to use "extreme caution" in the battle.
But Libya's Deputy Foreign Minister Khaled Kaaim told reporters that Ban's statement was "unacceptable" for not singling out NATO for "being responsible for civilian deaths in Libya" since the revolt broke out in mid-February.
Kaaim on Saturday also denied the rebels had entered the eastern oil hub of Brega almost six months after rising up against Kadhafi's forces, inspired by revolts in Egypt and Tunisia.
They vowed on Friday that in "a few days" they plan to retake Brega on the Gulf of Sirte.
Rebels, backed by NATO helicopters, have tried for the past three weeks to seize the vital port 240 kilometres (140 miles) southwest of their eastern stronghold of Benghazi.
Tripoli last week accused NATO of killing 85 people, including women and children, with air strikes on the village of Majer, south of the disputed city of Zliten, charges NATO denied.