BAGHDAD — Hezbollah instructors trained Shiite militiamen at remote camps in southern Iraq until three months ago when they slipped across the border to Iran — presumably to continue instruction on Iranian soil, according to two Shiite lawmakers and a top army officer.

The three Iraqis claim the Lebanese Shiites were also involved in planning some of the most brazen attacks against American-led forces, including the January 2007 raid on a provincial government compound in Karbala in which five Americans died.

The allegations, made in separate interviews with the Associated Press, point not only to an Iranian hand in the Iraq war, but also to Hezbollah’s willingness to expand beyond its Lebanese base and assume a broader role in the struggle against American influence in the Middle East.

All this suggests that Shiite-dominated Iran is waging a proxy war against America to secure a dominant role in majority-Shiite Iraq, which has supplanted Lebanon as Tehran’s top priority in the Middle East.

“The stakes are much higher in Iraq, where there is a Shiite majority, oil, the shrine cities and borders with Saudi Arabia,” said analyst Farid al-Khazen, a Christian Lebanese lawmaker whose party is allied with Hezbollah.

“The big story is Iraq, and the Americans unwittingly opened it up for the Iranians” by their invasion in 2003, Mr. al-Khazen said.

The allegations come as America and Iran are engaged in a showdown over Tehran’s nuclear program and each country’s role in Iraq.

Iran, Hezbollah’s mentor, denies giving any support to Shiite extremists in Iraq.

But the three Iraqis who spoke to the AP said the Iranians prefer to use Hezbollah instructors because as Arabs, they can communicate better with the Iraqi Shiites and maintain a lower profile than Farsi-speakers from Iran.

For Hezbollah, a high-risk role in Iraq could give the Lebanese movement leverage with America and broaden its appeal within the Arab world where anti-American sentiment remains strong.

Iraqi officials have said little about a Hezbollah role in this country. However, President Talabani told America-funded Alhurra television this week that “there have been several occasions” when Hezbollah members or those who “claim to belong to Hezbollah” have been detained in Iraq.