Taking the coastal town would be an important step for Libya's new rulers. It would bring them closer to finally gaining control of the whole country almost two months after their fighters seized the capital Tripoli.

Forces loyal to the ruling National Transitional Council (NTC) are under pressure to make swift progress on the battlefield but heavy resistance by Qaddafi loyalists has prevented them from taking the city for weeks.

Columns of black smoke rose above Sirte's skyline as NTC forces fired tank shells and rounds of artillery toward the city center from their positions to the east of Sirte.

Thousands of civilians have fled Sirte as fighting has intensified, describing increasingly desperate conditions for those inside the city. Many were caught off guard by Friday's assault and fled in panic as explosions boomed around the city.

"There are strong strikes in all directions. Today we will finish it. God willing, today we will capture Sirte," said Col. Ahmed El-Obeidi, an NTC commander.

NTC trucks with ammunition and artillery batteries were brought forward to the eastern front line, 1.2 km (0.8 miles) from the city center.

Along with the desert town of Bani Walid, Sirte is one of the last strongholds still controlled by Qaddafi loyalists.

Doctors at a field hospital east of Sirte said they heard a huge explosion inside the city after midnight. Col. Obeidi said the explosion occurred after his forces hit a weapons storage facility belonging to Qaddafi supporters.

Qaddafi loyalists who pulled back to Sirte after losing control of other cities are putting up fierce resistance. They have been mainly using sniper fire and rocket-propelled grenades to prevent NTC forces from entering the city center. The prolonged fighting has raised concerns about civilians in the coastal city of 75,000 people.

At a field hospital several kilometers west of Sirte dozens of ambulances brought in at least 50 fighters and civilians wounded in the fighting. Most were seriously wounded.

Medical workers tried to save an unresponsive woman. A child with a wounded leg sat on the floor. A helicopter landed near the hospital to carry some of the wounded to hospitals in Misrata, which lies west of Sirte.