The US says it has evidence linking a Jordanian-born al-Qaeda suspect to Tuesday's devastating attacks in Iraq which killed up to 271 people.
Shias blame the US for failing to protect them
The American commander in Iraq, Gen John Abizaid, hardened the US line on Abu Musab al-Zarqawi in testimony to Congress in Washington.
A letter purporting to be from al-Qaeda has denied any involvement.
The blasts on Tuesday came at the climax of a Shia festival and marked Iraq's bloodiest day since the war.
Three days of national mourning are being observed in Iraq where thousands of people have taken part in the first funerals.
Fifteen people were arrested in Karbala after the blasts, including five people believed to be Iranians.
Top US administrator for Iraq Paul Bremer blamed infiltrators for the attack and said new measures would be taken to increase security on Iraq's borders.
"There are 8,000 border police on duty today and more are
on the way," Mr Bremer said.
"We are adding hundreds of
vehicles and doubling border police staffing in selected
areas. The United States has committed $60m to
support border security."
One day after the attacks, the death toll rose sharply, although there is a discrepancy between figures given by Iraqis and US officials.
"The number of martyrs from the two cities as of this afternoon is 271," Iraqi Governing Council President Mohammed Bahr al-Uloum told a news conference in Baghdad on Wednesday.
US officials earlier said 117 had died.
"We have clear intelligence that ties Zarqawi to this attack," Gen John Abizaid told the House of Representatives' Armed Services Committee.
The head of the US Central Command did not give any details of the role in the attacks by the Jordanian-born suspect.
"The level of organisation and the desire to cause casualties among innocent worshippers is a clear hallmark of the Zarqawi network," Gen Abizaid said.
"We also have intelligence that shows there are some linkages between Zarqawi and former regime elements, particularly the Iraqi intelligence services," he added.
US officials say a letter from Mr Zarqawi urging attacks on Shia Muslims was intercepted last month.
MAJOR ATTACKS SINCE 1 MAY
2 Mar 2004: Up to 271 Shias killed in Baghdad and Karbala during the Ashura religious festival
1 Feb 2004: At least 101 killed by twin suicide bombings during celebrations in Kurdish city of Irbil
18 Jan 2004: 18 reported killed outside coalition HQ, Baghdad
14 Dec 2003: Car bomb at police station kills 17 in Khalidiya, west of Baghdad
12 Nov: 26 die in suicide attack on Italian base in Nasiriya
2 Nov: 16 US soldiers die as Chinook helicopter downed
27 Oct: Red Cross and other buildings in Baghdad bombed, more than 30 killed
29 Aug: Mosque near Najaf bombed, at least 80 dead including top Shia cleric
19 Aug: UN headquarters in Baghdad bombed, 23 killed including head of mission
List covers attacks since US declared war effectively over
In London, journalists at the al-Quds al-Arabi newspaper, which received a letter signed by a group said to be linked to al-Qaeda - the Abu Hafs al-Masri Brigades - said they believed the document, denying responsibility, was genuine.
Tens of thousands of people turned out in Baghdad and Karbala to bury the dead.
Chanting "God is greatest", the Karbala mourners carried aloft the coffins draped with palm leaves and Iraqi flags.
A top Shia cleric, Ayatollah al-Sistani, criticised the US for inadequately securing the country's borders from foreign attackers.
A US official said terrorists were believed to have crossed into Iraq with Iranian pilgrims planning to celebrate Ashura - a commemoration of the death of Imam Hussein in 680.