AJDABIYA, Libya — Government forces shelled the rebels’ main front-line outpost today, forcing fighters to pull back and open fire with their own heavy weapons to try to hold the town.

As the shelling intensified, a major blast occurred on the outskirts of the city and threw up a mushroom cloud. It was not immediately clear whether it was a NATO airstrike or part of the ground battle, but the shelling ceased soon afterward.

The government attacks on Ajdabiya quickly changed the fortunes of rebels who had earlier sent units deeper toward the strategic oil port of Brega, 40 miles from Ajdabiya, and captured two soldiers loyal to Moammar Gadhafi.

It’s unclear whether government troops would make a push of their own into the eastern town of Ajdabiya, which has been nearly deserted by civilians. But taking control of the town would open the way to the rebel stronghold of Benghazi and the eastern port of Tobruk near the Egyptian border.

Most rebel forces pulled back to Ajdabiya when the shelling began, but later appeared to attempt a counter-offensive with heavy machine guns and rockets. Black smoke rose from parts of the town and some buildings were ablaze.

Earlier, rebels had pushed deeper toward Brega, a key prize in the back-and-forth battles with government forces.