Washington/Baghdad — The US is deeply concerned over reports of Iran moving ballistic missiles into Iraq, US secretary of state Mike Pompeo said.

If found to be true, he tweeted, it would be a "gross violation of Iraqi sovereignty" and a breach of a UN resolution.

Pompeo was responding to a Reuters report that Iran had provided missiles to Shiite proxies in Iraq in the past few months.

The country was also said to be helping these groups develop the capacity to build missiles in Iraq to deter attacks on its interests in the Middle East, Reuters reported, citing unidentified Iranian, Iraqi and Western intelligence officials.

Iranian foreign ministry spokesman Bahram Qassemi on Saturday rejected the allegations, calling them "unfounded" and "absurd".

Qassemi said the claim was intended to "fuel fear in regional countries in continuation with a policy to demonise Iran", the state-run Mehr news agency reported.

The US has often accused Tehran of violating UN Security Council Resolution 2231, which endorsed a nuclear deal in 2015 between Iran and world powers.

The resolution calls on Iran not to undertake activities related to ballistic missiles that can be capable of delivering nuclear weapons.

Reuters reported that, according to three Iranian officials, two Iraqi intelligence sources and two Western intelligence sources, Iran has transferred short-range ballistic missiles to allies in Iraq over the past few months.

Five of the officials said it was helping those groups to start making their own.

"The logic was to have a backup plan if Iran was attacked," one senior Iranian official told Reuters. "The number of missiles is not high, just a couple of dozen, but it can be increased if necessary."

Iran has previously said its ballistic missile activities are purely defensive in nature.

The Zelzal, Fateh-110 and Zolfaqar missiles in question have ranges of about 200km-700km, putting Saudi Arabia’s capital Riyadh or the Israeli city of Tel Aviv within striking distance if the weapons were deployed in southern or western Iraq.

The Quds Force, the overseas arm of Iran’s powerful Islamic Revolutionary Guard Corps (IRGC), has bases in both those areas.

Quds Force commander Qassem Soleimani is overseeing the programme, three of the sources said.

Western countries have already accused Iran of transferring missiles and technology to Syria and other allies of Tehran, such as Houthi rebels in Yemen and Lebanon’s Hezbollah.

Iran’s Sunni Muslim Gulf neighbours and its archenemy Israel have expressed concern about Tehran’s regional activities, seeing it as a threat to their security.

Israeli officials did not immediately respond to requests for comment about the missile transfers.

Israeli Prime Minister Benjamin Netanyahu said on Wednesday that anybody who threatened to wipe Israel out "would put themselves in a similar danger".

Bloomberg and Reuters