The United States has issued a very ominous warning to nine Iranian vessels suspected of carrying weapons to the Houthi rebels in the besieged north-African nation of Yemen. It has deployed the massive aircraft carrier USS Theodore Roosevelt and eight other combat vessels to waters off the embattled country to keep an eye on the shipment - and 'in response to the deteriorating security situation' there. The cargo ships' presence raised fears within the Saudi-led coalition which is helping Yemen's government fight off  Iranian-backed rebels known as the Houthis. The coalition of Arab countries backed by America continued its aerial assault on the rebels in the country today, killing at least 20 in the capital Sana'a. Their assault is aimed at crushing the Houthis who have already taken over regions in northern Yemen and are now trying to expand their gains throughout the rest of the country. Scroll down for video . Warning: The United States has deployed the aircraft carrier USS Theodore Roosevelt (pictured) off the coast of Yemen in response to the worsening security situation in the country . Armed response: The Roosevelt (back) and guided-missile cruiser USS Normandy (front) are among 12 American ships stationed in the waters . Keeping watch: The US fleet is keeping an eye on eight Iranian cargo vessels suspected of carrying weapons to the Houthi rebels in Yemen . Tactical: But Pentagon spokesman Colonel Steven Warren refused to say whether the US flotilla - armed with F18 fighter jets (pictured) - would intercept the fleet if it neared a Yemeni port . Dangerous: The Roosevelt (pictured) which carries around 5,000 crew and some 60 aircraft has been deployed to the Yemeni coast along with a group of three amphibious ships and two minesweepers . Bombardment: A Saudi-led coalition has continued to bomb Yemen today (pictured), killing at least 20 people in the capital Sana'a . Collateral: Residents in Sana'a inspect a bridge which was destroyed by a Saudi-led airstrikes targeting Houthi positions in the nearby city of Ibb . Battle: The cargo ships' presence raised fears within the Saudi-led coalition which is helping Yemen's government fight off Iranian-backed rebels known as the Houthis (pictured) Pentagon spokesman Colonel Steven Warren said US ships are closely monitoring a convoy of nine Iranian cargo ships but refused to say whether they would be intercepted if they neared a Yemeni port. Warren said: 'We don't know what the Iranian convoy of ships plans to do, but we are watching them... By having American sea power in the region, we preserve our options.' He claimed there has been no communication with the ships and said reports the Americans were preparing to intercept the convoy were 'a bit over-cranked'. The convoy's presence 'is one of the factors' in any US naval engagement but not the only one, Warren added. Nine combat vessels were among 12 US ships in the region stretching from the Red Sea to the Gulf of Aden and the Arabian Sea. The warships included the Roosevelt which carries around 5,000 crew and some 60 aircraft, a group of three amphibious ships and two minesweepers. Three resupply ships are in the area to support combat vessels. Colonel Warren added: 'They are operating in the Arabian Sea and the Gulf of Aden with the very clear mission to ensure that shipping lanes remain open, to ensure there's freedom of navigation through those critical waterways and to ensure there's maritime security.' The Iranian vessels were suspected of carrying arms and equipment destined for the Huthi rebels, United States defense officials said on Monday. But they suggested it was more likely that Saudi Arabia, Egypt or one of their allies would intercept the vessels if necessary. An arms embargo was imposed on the Iranian-backed Houthi forces by the UN Security Council earlier this month. The Saudi-led intervention was launched in late March to help pro-government forces fighting the insurgents. Reinforcements: Nine combat vessels are among 12 US ships in the region stretching from the Red Sea to the Gulf of Aden and the Arabian Sea . Take off: A shooter launches an F/A-18F Super Hornet (pictured) off the flight deck of USS Theodore Roosevelt stationed off the coast of Yemen . Force: The US fleet's deployment comes after a U.N. Security Council resolution last week imposed an arms embargo on Houthi leaders . Suspcious: The Iranian vessels being watched by the US fleet (pictured) were suspected of carrying arms and equipment destined for the Huthi rebels, United States defense officials said on Monday . Warning: Pentagon spokesman Colonel Steven Warren said: 'We don't know what the Iranian convoy of ships plans to do, but we are watching them' Intense ground fighting and airstrikes have pushed the Arab world's poorest country to the brink of collapse. Airstrikes hit a gathering of the Houthis in the western of Ibb today and killed 20 fighters, security officials on the ground have said. Although the rebel-controlled Interior Ministry said 84 people were killed across the country in Monday's airstrikes, the casualty figures could not be independently confirmed. Saudi Arabia has accused Iran of arming the Houthis, a claim both Tehran and the rebels deny even though the Islamic Republic has provided political and humanitarian support to the Shiite group. The Shiite nation of Iran has long accused Saudi Arabia of supporting Sunni militants, including the Islamic State group in Syria and Iraq. In remarks Tuesday, Iranian President Hassan Rouhani said he airstrikes in Yemen were prompted by the Sunni kingdom's failures elsewhere, causing what he called a 'mental imbalance.' Speaking to reporters before heading to Indonesia, Rouhani mocked Saudi Arabia by calling it a country with dashed dreams in Iraq, Syria and Lebanon, adding: 'All the failures have accumulated and caused mental and emotional imbalance for that country.' The remarks came a day after U.S. Navy said aircraft carrier USS Theodore Roosevelt was steaming toward the waters off Yemen to beef up security and join other American ships that are prepared to intercept any Iranian vessels carrying weapons to the Houthi rebels. Operation: Saudi Arabia (soldier pictured) has accused Iran of arming the Houthis, a claim both Tehran and the rebels deny . Retaliation: Meanwhile Iran's president said the airstrikes in Yemen (pictured) were prompted by Saudi Arabia's failures elsewhere, causing what he called a 'mental imbalance.' War: As Saudi forces continued a ground operation against Houthis on Monday - launching mortar shells (pictured) - the rebel-controlled Interior Ministry said 84 people were killed across the country .
United States has deployed the aircraft carrier USS Theodore Roosevelt and 11 other ships off the coast of Yemen .
Nine combat vessels are monitoring Iranian vessels suspected of carrying weapons to Houthi rebels in the country .
Pentagon spokesman said they are monitoring the nine cargo ships but refused to say whether they would engage .
Meanwhile intense fighting between Iranian-backed rebels and Saudi-led coalition rages on in the embattled nation .