Iraqi Prime Minister Nouri Maliki has arrived in Iran for talks which are expected to concentrate mainly on security and co-operation.
Mr Maliki is leading a high-ranking delegation to Tehran
Mr Maliki's visit comes days after Iran and the Unites States held a third round of talks in Baghdad on improving Iraqi security.
Iran and Iraq fought an eight-year war in the 1980s.
But ties between the mainly Shia Iran and the Shia-dominated Iraqi government have become increasingly close.
Mr Maliki can expect strong expressions of support from Iran and offers of further help to quell the ongoing violence in Iraq, says the BBC's Pam O'Toole.
'Help on border'
The Iraqi ambassador to Tehran, Mohammad Majeed al-Sheikh, explained what Iran could do to bolster Iraq's security.
"We have a common border with Iran, nearly 1,400km (875 miles), which is the longest border we have with a neighbouring country. We don't think we're able, or rather we don't have the capabilities, to control the border," he told the BBC.
"Until our armed forces and border guards are strong enough, we believe we need the Iranians to help us do that at this stage and to stand by us.
"We also need them to talk to some neighbouring countries with whom Iran has strong relations to maintain security within Iraq. As you know, some of the terrorists infiltrate into Iraq from neighbouring countries."
Baghdad has also expressed hope that the ongoing talks between Washington and Tehran about Iraqi security could be held at a higher level, and the Iranian and Iraqi delegations may well discuss the next steps in that dialogue.
Mr Maliki is visiting Iran at a time of increasing domestic and international pressure on his government.
Most Sunni members of his cabinet have withdrawn, leading to speculation about his political future; and Washington has been increasingly critical of his administration's record.
Although Iran is strongly opposed to the presence of US forces in Iraq, it is anxious to prevent the country disintegrating, our correspondent adds.
It has strong historical ties with Iraqi Shia groups and would like a relatively strong Shia-dominated government to remain in power there.
US officials have recently accused Iran of stepping up its support for Iraqi militias - allegations Iran has always denied.
But Iraqi President Jalal Talabani said this week Iran had always played a constructive role in helping Baghdad to restore security and stability.