Iran has heavily-armed allies across the Middle East that the country could call upon if the government chooses to retaliate against the U.S for an airstrike that killed one of Tehran's top generals early Friday morning.

The network of pro-Iranian supporters trained by Tehran was set up by Gen. Qassem Soleimani who was killed by an airstrike ordered by President Trump at the Baghdad International Airport. The supporters include tens of thousands of fighters in Iraq, Syria, Lebanon, Gaza and Yemen.

After the airstrike, Khamenei warned that a “harsh retaliation is waiting” for the U.S.

IRAN VOWS 'HARSH RETALIATION' AFTER US AIRSTRIKE KILLS IRANIAN GEN. QASSEM SOLEIMANI

Iran has trained and financed thousands of Shiite militias in Iraq who helped fight the U.S. after 2003, including Asaib Ahl al-Haq, Kataeb Hezbollah and the Badr Organization. All three groups have close ties to Soleimani.

Abu Mahdi al-Muhandis, the leader of Kataeb Hezbollah, was killed along with Soleimani in the airstrike.

Hezbollah, strategically located on the border with Israel in Lebanon, has an arsenal of thousands of rockets and missiles. The group was created by the Iranian Revolutionary Guard during Lebanon’s civil war in the 1980s to fight Israel.

Iranian-backed Houthi rebels in Yemen who took the capital, Sanaa, in 2014, have been fighting against a Saudi-led coalition in a civil war that has cost more than 100,000 lives in the country, according to The Armed Conflict Location and Event Data project. Last year the Houthis attacked a critical Saudi oil pipeline.

Palestinian militant groups in Gaza, including Hamas and the Islamic Jihad group, have long been supported by Iran.

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Although Iran broke with Hamas after the Arab Spring in 2011, the country is believed to have continued military support for the group. However, Hamas is financially strapped and gets most of its funding from Qatar, making it less likely it would support Tehran in a regional war.

The Associated Press contributed to this report