(Adds U.N. secretary-general, paragraphs 6-7)

By Gul Yousafzai

QUETTA, Pakistan, Feb 2 (Reuters) - Gunmen abducted an American official working for the U.N. refugee agency in Pakistan on Monday and killed his driver, the latest in a string of attacks on foreigners in the country.

John Solecki, head of the United Nations High Commissioner for Refugees (UNHCR) office in the southwestern city of Quetta, was on his way to his office when gunmen intercepted his vehicle and opened fire, killing his driver.

"We strongly condemn this attack on humanitarian workers in Pakistan who have been doing their utmost to deliver their humanitarian mission," the United Nations said in a statement.

There had been no claim of responsibility, Quetta police official Wazir Khan Nasir said.

Solecki's vehicle crashed into a wall after the gunman opened fire from a car.

U.N. Secretary-General Ban Ki-moon's press office issued a statement condemning Solecki's abduction and the killing of his driver. He also demanded Solecki's immediate release.

"Such acts are aimed not only against U.N. personnel, but also against those they serve selflessly and with dedication," Ban said in the statement.

U.N. High Commissioner for Refugees Antonio Guterres said the agency was doing all it could to secure Solecki's release, and expressed sympathy to the family of the driver, Syed Hashim, who had worked for UNHCR for 18 years and leaves a wife and four children.

"The targeting of those who help the world's most vulnerable people leaves all of us deeply shocked and saddened," Guterres said in a statement from Geneva.

The Pakistani Foreign Ministry condemned what it called a "dastardly terrorist act" and said all measures were being taken for Solecki's safe recovery.

HEIGHTENED SECURITY

Security had been stepped up at the main crossing point into Afghanistan at Chaman, 100 km (60 miles) northwest of Quetta, and at other points along the border to stop the kidnappers from taking their hostage into Afghanistan, another police official said.

Quetta is the capital of Baluchistan province, which borders Afghanistan and Iran.

Both Taliban and separatist Baluch militants operate in the province where the UNHCR is helping Afghan refugees.

Incidents of kidnapping have increased in Pakistan over the past year, especially in the northwest, on the Afghan border, where Islamist militants have abducted foreigners.

Pakistani Taliban militants were believed to be responsible for the kidnapping of a Chinese engineer, a Polish engineer and two Afghan diplomats. All four were kidnapped in the northwest months ago and are still being held.

Gunmen shot dead an American aid official outside his home in the northwestern city of Peshawar in November. The man's driver was also killed.

In August, a U.S. diplomat escaped unharmed when gunmen ambushed her vehicle in Peshawar.

The United Nations ordered the children of foreign staff to leave Pakistan in October after a suicide truck-bomb attack on the Marriott Hotel in Islamabad killed 56 people, including six foreigners.

The violence poses a problem for aid groups that want to increase help to the impoverished northwest, but are unable to send staff, in particular foreigners, into the area to implement and monitor projects because of security fears. (Additional reporting by Robert Birsel in ISLAMABAD and Jonathan Lynn in GENEVA; Writing by Augustine Anthony; Editing by Paul Tait)

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