Al Jazeera's Dan Nolan, reporting from Kabul, said that an Afghan official based in Lashkar Gah described the attack as the "worst fighting he's ever seen in the town".
Captain Mark Windsor, an Isaf spokesperson, told Al Jazeera that the military "does not do body counts".
"We do not try to figure out the number of insurgents killed ... once you get into the details or try to guess a number, then things might be different to what you said at the beginning," he said.
"What we do know is that this was a planned attack against the Afghan base, where they had tried to take over the compound. What followed was an operation to battle these insurgents."
Daud Ahmadi, a spokesman for the Helmand government, said 64 Taliban fighters were killed.
But his death toll could not be verified independently and journalists are not able to travel to the remote and dangerous battle sites.
Afghan officials have been known to exaggerate death tolls in the past.
In a second battle, in Helmand, Afghan and international troops retook the Nad Ali district centre - which had been held by fighters - during a three-day fight, Ahmadi said.
That battle, which also involved air strikes, ended on Saturday and resulted in the death of 40 Taliban fighters, officials said.
Afghan police and soldiers were now in control of the district centre.
Nato said its aircraft bombed fighters after they were seen gathering for a major attack, killing "multiple enemy forces".
"If the fighters planned a spectacular attack prior to the winter, this was a spectacular failure," Richard Blanchette, an Isaf spokesman, said.
Helmand is the largest drug-producing area in the world, and the region alone accounts for more than half of Afghanistan's production of opium poppies.
More than 90 per cent of the world's opium is produced in Afghanistan, and up to $100m of the trade's profits are used to finance the Taliban.
The violence has killed more than 4,700 people this year, according to an Associated Press tally of figures from Western and Afghan officials.