Four troops from the NATO-led International Security Assistance Force (ISAF) have been killed in a roadside bombing in eastern Afghanistan, the coalition said, following a rise in violence in the country in recent weeks.

ISAF gave no other details about the incident on Saturday, or the identities of those killed. Most of the ISAF troops serving in the hotly contested east, near the border with Pakistan, are Americans.

At least 220 foreign troops have been killed in Afghanistan so far in 2011, with 57 killed in May when the Taliban began their "spring offensive".

The Taliban vowed to attack foreign military targets, as well as the Afghan government and security forces, when they launched their offensive at the beginning of the traditional warm-weather fighting season in Afghanistan.

US commanders had warned of a spike in violence, saying that the Taliban could be expected to hit back after significant gains were made by mainly US NATO troops in the southern Taliban heartland of Helmand and Kandahar over the past year.

Drawdown of forces

Despite record violence, Washington has pledged to begin a gradual drawdown of its forces from next month as part of a process that will see all foreign combat troops leave Afghanistan by the end of 2014.

Separately on Saturday, Robert Gates, the US defence secretary, said military pressure on the Taliban could lead to "real opportunities" for peace talks with leaders of the Afghan fighters within a year, adding that the Taliban could not win militarily in Afghanistan.

Analysts and some US legislators have questioned the wisdom of starting to withdraw troops so soon after those gains were made and with violence rising steadily across Afghanistan.

Violence across Afghanistan hit its worst levels in 2010 since the Taliban were ousted by US-backed Afghan forces in late 2001, with record military and civilian casualties.

A total of 711 foreign troops were killed in Afghanistan in 2010, the deadliest year of the war, according to figures compiled by independent monitor www.icasualties.org and Reuters.

June 2010 was also the bloodiest single month of the war for foreign troops, with 103 killed. At least seven have been killed in the first four days of June this year.