Asked about accusations that Iran was interfering in the internal affairs of Iraq, al-Jaafari told Iranian state television on Thursday: "Such accusations are baseless and we do not agree with them at all."
"Relations between Iran and Iraq are currently very friendly and strong and expanding. We are proud of the situation," he said.
"Some people want to harm our existing friendly relations with Iran. But we will not let them do so. We are determined to expand our relations every day."
British Prime Minister Tony Blair said on Thursday that evidence pointed to Iran or its Lebanese Hizb Allah allies as the source of explosives used in roadside bombs in Iraq, although Britain did not have conclusive proof.
Saudi Arabia's Foreign Minister Saud al-Faisal has also criticised Iran's growing influence in predominantly Shia Muslim Iraq.
Tehran denies it helps fighters in Iraq. Al-Jaafari said an Iraqi cabinet minister would shortly visit Iran to follow up on a number of agreements signed between the two countries which have considerably improved relations since the fall of Saddam Hussein in 2003.
Under Saddam, Iraq and Iran fought a bitter 1980-1988 war in which hundreds of thousands of people died.