LASHKAR GAH, Afghanistan (Reuters) - Taliban insurgents killed at least 15 Afghan policemen when they overran two checkpoints in Helmand province in an attack on Tuesday that sharply increased pressure on the beleaguered provincial capital of Lashkar Gah, officials said.

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The attack on checkpoints in Gereshk, on the main highway through Helmand, a few kilometers to the north of the governor’s compound in Lashkar Gah, and Nad Ali, to the west of the town, underlined the growing pressure on security forces clinging on in the southern province.

“The situation is very critical near Lashkar Gah,” said a senior provincial security official, adding that 15 policeman had been killed.

“If the government does not act soon, there will be a disaster,” said the official who spoke on condition of anonymity.

Helmand, a Taliban heartland and the biggest source of Afghanistan’s opium, has been under pressure for months, with government forces pulling out of several districts to regroup around Lashkar Gah.

The province is one of the key battlegrounds for the Taliban, who launched a spring offensive last month, vowing to drive out the Western-backed government in Kabul and restore strict Islamic rule.

After briefly capturing the northern city of Kunduz last year, the insurgents, who control more territory than at any time since 2001, appear determined to gain control of a province to use as a base for their campaign.

Hundreds of American troops were sent to Helmand this year to bolster its defense with a beefed up training and advisory mission but a promised offensive by Afghan forces has not materialized.

After a few weeks of relative calm during the opium harvest, expected to be a bumper crop, fighting has picked up, underlining the fragile hold government forces have on the province.

“Over the past few days, the Taliban have been attacking security belts near Lashkar Gah,” said Bashir Shaker, a member of the provincial council.

He said 17 policemen had been killed in the latest fighting.

“The threat is becoming bigger minute by minute. If the government does not take action soon, we will witness the collapse of Lashkar Gah.”

British and U.S. forces struggled for years to control Helmand and many of the more than 450 British servicemen and women killed in Afghanistan lost their lives there.

In a statement, Taliban spokesman Qari Yousuf Ahmadi said the insurgents had overrun two checkpoints in Gereshk.

There has been heavy fighting in several other areas of Afghanistan, in provinces ranging from Kunduz, to Kunar in the east and Uruzgan and Ghazni in the center.