The US military says the insurgency is getting more effective
US President George W Bush has warned Iran and Syria not to interfere in Iraq in advance of next month's elections.
Mr Bush said he expected all of Iraq's neighbours, including Iran and Syria, to stop what he said was the flow of people and money into Iraq.
The influx was aimed at helping terrorists, he said.
His comments came on the first day of the election campaign, as at least seven people were killed and 30 injured by a bomb in the holy city of Karbala.
The blast at the gate to a major Shia shrine, the Imam Hussein mausoleum, was the first serious attack in the city for several months.
Sheikh Abdul Mehdi Karbalai, an aide to Iraq's most senior Shia cleric Grand Ayatollah Ali Sistani, was said to be among the wounded.
A spokesman for Ayatollah Sistani told al-Jazeera television station the bomb was probably an attempt to assassinate the cleric.
'Stop the flow'
Mr Bush gave the warning after Iraq's interim Defence Minister, Hazim Shaalan, accused Iran and Syria of orchestrating terrorist attacks and branded Tehran the "most dangerous enemy of Iraq".
Sistani's spokesman said the bomb might be an assassination bid
"We will continue to make it clear, to both Syria and Iran that, as will other nations in our coalition... that meddling in the internal affairs of Iraq is not in their interests," the US president said at a joint news conference with Italian Prime Minister Silvio Berlusconi.
"We expect people to work with the Iraqi interim government to enforce the border to stop the flow of people and money that aim to help these terrorists," he added.
Meanwhile a senior American general said the insurgency in Iraq had become "more effective".
Air Force Lt Gen Lance Smith said explosive attacks on US supply lines were slowing military operations and hindering the country's reconstruction.
Scheduled for 30 Jan
Voters to elect 275-member transitional assembly
Kurds also to pick 111-member autonomous parliament
Campaigning begins 15 Dec
230 parties running in about 80 blocs
Proportional representation based on party lists
Candidates must by over 30 and one third must be women
Militias, ex-top Baathists and current army officials barred
Balloting to take place in some 9,000 polling stations
He added that while there was influence and an intent to influence from Iran, the extent of it was "difficult to gauge".
As for Syria, the highest levels of government did not appear to have sanctioned such activity but there was a "significant amount" of both financial support and movement across the border of foreign fighters, he said.
On 30 January, voters will elect a 275-member assembly that will appoint a government and draft a constitution.
Prime Minister Iyad Allawi announced he would stand at the polls backed by a 240-member list of candidates from his Iraqi National Accord party, intended to have a broad appeal.