UPDATE 3-At Least 20 Killed in Suicide Bombing in East Afghanistan

(Updates toll)

By Obaid Ormur

PULE ALAM, Afghanistan, June 25 (Reuters) - A suicide car bomber killed at least 20 people, and possibly as many as 35, in an attack at a hospital in a remote district of eastern Afghanistan on Saturday, government and security officials said.

Estimates of the casualties, which included patients and medical staff, varied widely in chaotic scenes outside the hospital in the remote Azra district of eastern Logar province, which is just south of Kabul.

Dozens more were wounded in one of the worst attacks this year.

Deen Mohammad Darwish, a spokesman for the Logar provincial government, said as many as 35 people were killed, although Afghanistan's Interior Ministry put the death toll at 20.

"The exact target is still not clear," Interior minister deputy spokesman Najib Nikzad said.

President Hamid Karzai condemned the attack in which he said "tens of civilians" were killed.

Taliban spokesman Zabihullah Mujahid denied responsibility and said the Islamist insurgents never attack hospitals.

Tensions have flared over civilian casualties, with insurgents and the Afghan government alike criticising NATO-led forces for killing innocent Afghans while hunting for militants.

United Nations figures however show that insurgents are responsible for three-quarters of civilian deaths.

Military and civilian casualties hit record levels in 2010, the most violent year of the war since U.S.-backed Afghan forces toppled the Taliban in late 2001.

This year is following a similar trend, with violence growing across Afghanistan since the Taliban announced a spring offensive at the beginning of May.

U.S. commanders had already said they expected a rise in attacks as insurgents hit back after U.S. and NATO forces made gains during operations in the Taliban heartland in the south over the past 18 months.

The United Nations said two weeks ago that May had been the deadliest month for civilians since it began compiling statistics four years ago.

It said it had documented 368 "conflict-related" civilian deaths, 82 percent of them caused by insurgents.

The latest violence comes as seven areas across the country prepare to take over security responsibilities from the NATO-led International Security Assistance Force (ISAF) fighting an increasingly unpopular war that has dragged on for 10 years.

That will coincide with the start in July of a gradual drawdown of U.S. troops. U.S. and NATO troops plan to hand over security responsibility for all of Afghanistan by the end of 2014, although critics warn the handover date is premature.

While most of the ISAF gains have been in Taliban strongholds in the south, the insurgency in eastern provinces like Logar and Kunar near the border with Pakistan is much more fragmented.

Taliban fighters often slip across the border from safe havens in Pakistan's largely lawless northwest but other groups like the al Qaeda-linked Haqqani network also operate extensively in the area. (Additional reporting by Hamid Shalizi; Writing by Alistair Scrutton; Editing by Sugita Katyal)