A bomb in Pakistan's commercial capital Karachi wounded at least 19 people on Saturday, police said, in an apparent attack on Shi'ite Muslims ahead of one of their most important mourning rituals.

The explosion took place outside a Shi'ite Muslim gathering just before the start of Ashura, the Shi'ite calendar's biggest event, which has become a flashpoint for deadly attacks by Sunni militants over recent years.

Senior Police Superintendent Raja Umer Khattab said two policemen were wounded in the blast caused by a homemade bomb. It looks like a remote controlled or a time device as the blast took place along the roadside, he told Reuters. Police suspected earlier that it was a car bomb.

Police have stepped up security in the run-up to Ashura, whose climax is on Monday. The event draws thousands of people onto the streets in cities across Pakistan.

Pakistani authorities are already struggling to show they can contain militants who have killed hundreds of people in bombings since an offensive launched on one of the militants' strongholds in October.

Earlier on Saturday an official said that the Taliban had planned to use the five Americans, now detained in Pakistan, to carry out attacks inside the U.S. allied country.

The northwest city of Peshawar has been hit hardest by the recent spate of bombings but the militants have extended their reach as far a Karachi and the capital Islamabad.

DETAINEES FROM AMERICA

The case of five Americans detained in Pakistan who are suspected of using the Internet to contact Taliban insurgents suggests the task of containing militants could get more complicated for Pakistani authorities.

Washington is pushing Pakistan to root out militants who cross the border to fight U.S. troops in Afghanistan.

Usman Anwar, police chief in Sargodha, where the Americans were detained, said emails had revealed plans for the young men from Virginia to travel to a Pakistani nuclear power plant.

We believe that they were supposed to be used inside Pakistan, Anwar told Reuters by telephone.

In their last email to the Taliban, we found they mentioned the Chashma Nuclear Plant and that's why they were going to Mianwali (district).

They were arrested in Sargodha, home to one of Pakistan's biggest airbases, 190 km (120 miles) southeast of Islamabad. Two are of Pakistani ancestry, one of Egyptian, one of Yemeni and one of Eritrean and all may face terrorism charges.

Their alleged activities illustrate how easy it is for anyone to pursue dreams of joining militant jihad in countries such as Pakistan through cyber channels.

Some analysts say the issue reflects a new strategy by militants to try to avoid tighter security measures by forming networks on the Internet.

Pilotless U.S. drone aircraft attacks on suspected militants on Pakistani soil have underscored growing U.S. security concerns in a strategic region, where failure to bring stability could harm Barack Obama's U.S. presidency.

But the strikes have infuriated many Pakistanis and more pressure on Islamabad to root wipe put militants could backfire by spreading already strong anti-American sentiment.

A drone aircraft attacked a suspected militant target in northwest Pakistan on Saturday, killing five people, intelligence officials said. Two missiles hit a house in North Waziristan in the attack. Two people were wounded, the officials said.

The Taliban, which rejects ties to Washington, want to impose their harsh version of Islamic rule in Pakistan and have made clear how they would do it -- public whippings and hangings of those deemed immoral. Scare tactics are often used.

Local officials said on Saturday militants killed Gul Mohammad, a commander of an anti-Taliban militia, and dumped his beheaded body on a roadside in the Bajaur tribal district on the border with Afghanistan. A note was left warning that others could suffer the same fate, the officials said.

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