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Fears are growing for civilians caught up in fighting between government forces and the Taliban in Afghanistan's southern Helmand province.

There have been several days of clashes as Afghan forces, supported by US air strikes, try to defend Helmand's strategically important capital, Lashkar Gah, from a Taliban assault.

It is estimated that about 35,000 people have so far fled their homes.

Amnesty International called on both sides to give civilians "safe passage".

The group said the fighting had taken out power in Lashkar Gah and shut down telecommunication networks, and that all exit routes from the city had been blocked.

"The situation for civilians in Lashkar Gah is grave and could deteriorate rapidly in the coming days. Tens of thousands of people are trapped in the middle of a bloody battle that shows no sign of abating," Omar Waraich, head of South Asia at Amnesty International, said in a statement.

The United Nations Assistance Mission in Afghanistan earlier called on both sides to "take all feasible measures to protect civilians", including "safe paths for those wishing to leave".

Aid agencies are reporting that many civilians are now sleeping on the streets without shelter.

One family told the ´óÏó´«Ã½ earlier this week that they left their home in Lashkar Gah with only the clothes they were wearing. Others said they feared they might die from hunger, while staff at local hospitals said they had admitted dozens of casualties.

Pregnant women are among those to have been injured in the fighting in Helmand.

One 18-year-old woman told Reuters news agency that she lost her baby when she was shot in the stomach while caught in the cross-fire in Gereshk district this week.

"I hadn't even chosen a name for him," she said. "My innocent child, gone forever."

The battle over Lashkar Gah marks the first big Taliban offensive since historic peace talks between the two sides began last month.

Correspondents say Taliban actions on the battlefield are again raising questions about their commitment to the negotiating table.

Two Afghan military helicopters transporting wounded soldiers collided on Wednesday, killing nine people on board. Afghanistan's defence ministry said the collision was due to "technical issues".