A roadside bomb ripped through a crowded Afghan bus Wednesday, killing up to 25 civilians in a southwestern province on the Iranian border in one of the deadliest such attacks in months.

The bomb exploded as the bus travelled on a highway through Nimroz province, provincial governor Ghulam Dastgir Azad and the interior ministry said.

Afghan officials blamed the attack on the Taliban, which is fighting to overthrow the Afghan government and evict nearly 150,000 foreign troops in a nine-year insurgency. The militant group denied involvement.

Azad said more than 20 civilians were killed in the blast. The interior ministry in Kabul put the death toll at 25, with more than 20 others injured.

"The bomb was planted by the enemies because this morning a coalition convoy was supposed to cross the area," Azad said, referring to NATO forces.

"Coalition forces helped us to evacuate the injured."

Afghan civilians are bearing the brunt of the war, suffering death and injury in crossfire, suicide attacks and from roadside bombs or improvised explosive devices (IEDs), which kill and maim indiscriminately.

Zabihullah Mujahed, a Taliban spokesman, called AFP from an unknown location to deny that the militant group was behind the bombing. Taliban are known for denying attacks involving civilian casualties and for rampant propaganda.

"We are not responsible for this bombing. This is the work of the NATO. They've done this in a bid to divert attentions from Sangin incident," Mujahed told AFP, referring a rocket attack that killed 52 civilians last Friday.

Afghan authorities and NATO-led troops are investigating the incident in Helmand province's Sangin district. NATO have denied involvement in the strike.

Surveys show that most civilian casualties in the war are caused by Taliban attacks. Roadside bombs are the militants' main weapon.

A NATO spokesman said civilian casualties caused by coalition's operations had fallen by nearly 11 percent in recent months while the number of non-combatants killed in militant attacks had increased by nearly 40 percent.

The force was doing "all we can" to protect civilians, German Brigadier General Josef Blotz, spokesman for NATO's International Security Assistance Force (ISAF), told reporters.

President Hamid Karzai issued a statement "strongly" condemning the bombing.

Thousands of Pentagon files leaked earlier this week indicated that civilian deaths have been covered up and that Iran is funding the Taliban eight years after the 2001 US-led invasion ousted the radical Islamist regime from power.

Iranian President Mahmoud Ahmadinejad denied those charges in an interview, telling CBS television: "We do not support any group" but calling the US-led invasion the "root cause" of the violence in Afghanistan.

Many Afghan people also blame the violence on the presence of the foreign troops.

US and NATO deployment is set to peak next month, as an extra 30,000 troops ordered into Afghanistan by President Barack Obama have mostly arrived in a bid to speed the end of the war.

Most of the troops have been sent to the volatile Taliban heartland in the southern provinces of Helmand and Kandahar, though the US is also beefing up other regions as the influence of the insurgency spreads.

A report by the US National Bureau of Economic Research this month analysed data on military clashes and incidents in which more than 4,000 civilian casualties in a 15-month period.

It found that civilian casualties were fuelling the insurgency, and that rules of engagement aimed at cutting the number of ordinary Afghans killed and injured also led to a decline in attacks by insurgent fighters.