At least 19 Afghan civilians were killed when their bus was hit by a roadside bomb in southern Afghanistan today.
Five children were among the dead on a road in Helmand province's volatile Sangin district - a Taliban stronghold.
The blast comes a day after twin bombings on Shiite Muslims celebrating the holy holiday of Ashoura left 60 dead and sparked fears that attacks in Afghanistan might be taking on a sectarian dimension for the first time.
Afghan president Hamid Karzai cancelled a planned visit to Britain to return to Afghanistan after yesterday's attacks.
Yesterday's bombings, the largest of which targeted a Shia Muslim shrine in the capital Kabul, wiped out any residual optimism from an international conference about the future of Afghanistan, held on Monday in Germany, and refocused attention on the fragile Afghan security situation.
Afghans have previously been spared the large-scale sectarian attacks that regularly trouble Iraq and neighbouring Pakistan, but now face the grim prospect of a new type of bloodshed being added to the dangers of daily life.
"The reason for President Karzai's trip cancellation to Britain is the terrorist attacks on Ashura in Kabul, Mazar-i-Sharif and Kandahar which killed and wounded many participants," Mr Karzai's office said in a statement.
The Nato-led International Security Assistance Force (ISAF), responsible for security across much of the country, says it is winning the war against the Taliban.
But if yesterday's bombing sets a precedent for violence between the Sunni Muslim majority and the Shia minority, it would severely stretch army and police resources.
At a funeral ceremony today for victims of the attack, hundreds of Shia Muslims bore aloft the bodies of the dead, chanting that because they had been killed at a Muslim ceremony, they had died in the name of the Prophet Mohammad.
"We were sacrificed for you," they shouted.
Some Shia Muslims said immediately after the Kabul blast that police had not done enough to protect them. Hundreds of worshippers had gathered to mark the festival of Ashura at a shrine in central Kabul when a suicide bomber struck.
Among those killed in yesterday's attacks was a US citizen, the American embassy in Kabul said in a statement. It gave no further details.
At the German conference, the Afghan government's Western backers, who have spent billions of dollars on the country since the US-led overthrow of the Taliban government in 2001, pledged to support the country beyond the end-2014 deadline for the withdrawal of foreign combat troops.
Afghanistan has said that it will not be able to afford the army and police force it needs after 2014 without international help, and yesterday's attack is likely to reinforce fears about the ability of Afghan forces to cope with violence after ISAF has fully handed over security.
Reuters