Iraqi army helicopters hit a jihadist convoy in eastern Syria Sunday, killing at least eight, in a show of strength just days before the country's first general election since 2010.

It was the first strike inside Syria claimed by Iraq since the three-year uprising against President Bashar al-Assad erupted in March 2011.

The conflict has spilt across the border, contributing to the most dramatic rise of violence in Iraq since the country's 2006-2008 sectarian war, with more than 600 people killed so far this month.

Meanwhile, Iraqis living overseas began voting in the parliamentary election in which Prime Minister Nuri al-Maliki is seeking a third term despite worsening sectarian tensions at home, rampant corruption and high unemployment. Polling day inside Iraq is Wednesday.

"The army struck eight tanker trucks in Wadi Suwab inside Syrian territory as they were trying to enter Iraqi territory to provide the Islamic State of Iraq and the Levant (ISIL) with fuel," interior ministry spokesman Brigadier General Saad Maan said.

ISIL emerged in Iraq in the wake of the US-led invasion in 2003, and later expanded into Syria during the uprising against Assad.

Maan said "there was no coordination with the Syrian regime" over the strike.

"Our responsibility now is to protect our border and to protect the border from the other side, because there is no protection from the other side," he said.

The targeted vehicles were apparently travelling to the western Iraqi province of Anbar, where ISIL has been battling Iraqi security forces.

ISIL fighters and other militants control the city of Fallujah in Anbar, a short drive from Baghdad, and security officials have voiced concerns the group is seeking to encroach on the capital as well.

Highlighting those concerns, ISIL carried out a double bombing at a Shiite political rally in Baghdad on Friday, killing 36 people.

The attack was the latest in a protracted surge in bloodshed in recent months.

More than 600 people have already been killed in April, including 10 who died in a series of attacks in Baghdad and northern Iraq on Sunday, officials said.

- Out of country vote -

The security forces' latest show of force came as voting opened for Iraqis living in 19 countries overseas, including substantial contingents in Britain, Sweden, Jordan, Iran and Germany.

"I want to see my country rise and assume its leading position," said Lina Moushtaq after she voted in Dubai.

Out of country voting is to continue on Monday. Prisons, hospitals and members of Iraq's security forces will also cast their ballots on Monday.

Maliki, accused by critics of monopolising power and targeting minority groups, is running for a third term and with a divided opposition, he remains the frontrunner, analysts and diplomats say.

The premier contends foreign interference is behind deteriorating security and complains he has been saddled with a unity government of groups that snipe at him in public and block his legislative efforts.

No single party is likely to win an absolute majority, however, and as in previous elections, coalition talks are likely to last months.

The month-long campaign has seen Baghdad and other cities plastered with posters and decked out in bunting, as candidates have taken to the streets, staged loud rallies and challenged each other in angry debates.

Attacks on candidates and their campaign staff and rallies have overshadowed the election, and parts of the country that have been outside government control for months will not see any ballots cast.

More than 9,000 candidates and 277 parties are competing for 328 seats in parliament, with upwards of 20 million Iraqis registered to vote in the first such election since US troops withdrew from Iraq in December 2011.