The capital had been relatively quiet recently
The UN has strongly condemned the bomb attack on the Indian embassy in the Afghan capital, Kabul, which killed 41 people and injured 141.
UN Secretary General Ban Ki-moon said Monday's attack could not be justified by any "political agenda or grievance".
Five embassy staff - India's defence attache, another diplomat, two security guards and an Afghan - were killed.
No-one has admitted being behind the attack, the deadliest in Kabul since the overthrow of the Taleban in 2001.
UN Secretary General Ban Ki-moon who is in Japan for the G8 summit, said he condemned the attack "in the strongest terms".
"No political agenda or grievance can justify such reprehensible means," Mr Ban said.
The UN Security Council described the attack as "reprehensible act of terrorism" .
The Afghan interior ministry has said it believed the attack was carried out "in co-ordination and consultation with an active intelligence service in the region".
It did not specify which intelligence service it suspected of involvement. But in the past, Afghanistan has accused Pakistani agents of being behind a number of attacks on its soil.
Pakistan Prime Minister Yousuf Raza Gilani denied his country was involved in the blast.
"Why should Pakistan destabilise Afghanistan? It is in our interest, a stable Afghanistan. We want stability in the region," he told reporters on the sidelines of a meeting in Malaysia's capital, Kuala Lumpur.
The bodies of the four Indians killed in the attack were flown to the Indian capital, Delhi, late on Monday by a special Indian air force plane.
RECENT ATTACKS IN KABUL
April 2008: Gun attack on parade attended by President Karzai
March 2008: Six people die in car bomb attack on coalition convoy
Jan 2008: Six people killed in Taleban attack on Serena hotel
Dec 2007: At least 13 people killed in a suicide car bombing
Sept 2007: Suicide bomb attack on bus kills 30 Afghan soldiers
June 2007: Bomb attack on Afghan police bus kills up to 35 people
India is one of Afghanistan's closest allies and leading donors - it has pledged to spend $750m on helping rebuild the country's shattered infrastructure.
The bombing dominated the front pages of Indian newspapers on Tuesday.
"Terror strikes India in Kabul," headlined the Indian Express newspaper.
"After the Kabul bombing, India must come to terms with an important question that is has avoided debating so far," the newspaper wrote.
"New Delhi cannot continue to expand its economic and diplomatic activity in Afghanistan while avoiding a commensurate increase in its military presence there."
Monday's attack happened when a suicide bomber rammed a car full of explosives into the gates of the Indian embassy.
Afghanistan has seen a sharp increase in violence, particularly in the south and east - and Taleban militants recently vowed to step up their attacks in the capital.
But the latest blast - in what was supposed to be a secure area of Kabul - will greatly concern Afghan government officials, says the BBC's Martin Patience in Kabul.