A homicide car bomber attacked a police checkpoint in eastern Afghanistan on Saturday, killing six people, police said, as the American envoy to the country said the U.S. would push for a "very significant increase" in the country's police force to improve security and stability.

Richard Holbrooke, who is the special envoy to both Pakistan and Afghanistan, said current plans to increase Afghanistan's national police from 78,000 to 82,000 over the next four years were "not sufficient."

Afghan police, who have less training and fewer weapons than Afghan and international soldiers, often bear the brunt of insurgent attacks.

"We need to devise programs which improve the Afghan government's capability to defend itself and that means considerably strengthening the Afghan national army and the Afghan national police," Holbrooke said during a debate on the Afghan war in Brussels. "We are looking in conjunction with our allies at a very significant increase."

European countries, not eager to send additional troops to fight, are currently being urged by Washington to provide military and police trainers, cash or other expertise to help stabilize Afghanistan. Holbrooke said the police force is now "riddled with corruption," and greater numbers of well-trained police would allow international troops in the country to focus on military operations rather than law enforcement.

President Barack Obama has pledged to send thousands of additional U.S. troops to Afghanistan this year in an attempt to improve the country's security and stability.

Eastern and southern Afghanistan have experienced the most violence in recent years from Taliban and Al Qaeda militants who regularly cross the border from sanctuaries in Pakistan to attack security forces and terrorize local civilians.

A NATO soldier was killed in a "hostile incident" in southern Afghanistan on Friday, and four Canadian troops serving with the NATO-led force were killed in two separate explosions, the alliance said.

The homicide car bombing that targeted the police checkpoint in the country's eastern Nangarhar province Saturday killed five civilians and one policeman, said police spokesman Gafor Khan. The blast also wounded four civilians and a policeman at the security post, set up to search cars entering Chaparhar district to celebrate the Persian new year.

Also in eastern Afghanistan, a bomb detonated by remote control killed five people near a shrine in Khost province as they celebrated the new year, known as Nowruz, said provincial police spokesman Wazir Pacha. The blast on the outskirts of Khost city wounded five people, he said.

Nowruz is celebrated in many countries, including Afghanistan, that were once part of the powerful Persian empire. The Taliban discouraged Afghan citizens from celebrating the holiday when they ruled the country in the 1990s because they deemed it un-Islamic. But people began celebrating Nowruz more openly again after the militant group was toppled by a U.S.-led invasion in 2001, when many Taliban fled to Pakistan.