Iran said on Wednesday it had sent two warships to the Gulf of Aden, heating up regional tension as an alliance led by rival Saudi Arabia continued its air campaign against Yemeni rebels.The ships were only intended to protect Iranian commercial shipping from pirates, Iranian naval commander Habibollah Sajjari said in remarks published by the official Islamic Republic News Agency.The move came as local media reported renewed air raids by the Saudi-led coalition on forces linked to Yemen's Houthi rebels - whom Saudi Arabia says are backed by Iran.The Saudi-led coalition is also enforcing a naval blockade of Yemen to prevent supplies reaching the Houthis.Local newspaper Aden al-Ghad reported airstrikes on pro-Houthi forces in Aden, the southern Yemeni city at the centre of current fighting, as well as neighbouring Lahj and Abyan provinces.Sources in the city - the last refuge of President Abd Rabu Mansour Hadi before he fled to Saudi Arabia in late March - reported fierce fighting as army units allied to the Houthis tried to advance on areas held by pro-Hadi militias.Residential buildings in militia-held areas were coming under shellfire from army positions, they said.Red Cross regional chief Robert Mardini wrote on Twitter that Aden was "devastated by three weeks of non-stop fighting" pitting the Houthis and allies, who control Sana'a, against Hadi loyalists.Army units linked to former president Ali Abdullah Saleh have played a key role in Houthi advances in the formerly independent south.The Houthis and Saleh have their support bases in the north, while Hadi is from the south, where local separatists and tribal fighters have joined forces with pro-Hadi militias.Aid agencies meanwhile warned that a humanitarian disaster was unfolding, with the conflict worsening existing food and water shortages.No food has been brought into Yemen since the Saudi-led coalition blocked its sea ports, Britain-based charity Oxfam said.That threatens catastrophe for a country where, even before the conflict, 10 million people did not have enough to eat and imports provided 80 to 100 per cent of key foodstuffs, Oxfam country director Grant Pritchard told dpa."We think that the humanitarian situation is deteriorating by the day and a humanitarian disaster is unfolding," Pritchard said."There is a lack of basic services and basic commodities. There are fuel and food shortages as well as (shortages of) medicine supplies," he added.The problems are compounded by deteriorating electricity and water supplies, which are hard to address as there is no security on the ground, Pritchard said."This is why Oxfam is calling on all parties to stop the violence and reach a peaceful solution," he told dpa."Humanitarian actors need access in a safe and secure manner. Fourteen out of 22 governorates [provinces] are affected by the conflict. We need this conflict to stop immediately," he said.The warning from Oxfam came as the Red Cross said it had sent a boat carrying medical supplies and a surgical team to Aden.Red Cross regional head of operations Robert Mardini wrote on Twitter that the organization had already managed to send supplies to two hospitals in northern and central Yemen.It was also about to get supplies to four health facilities in southern Yemen, scene of much of the recent fighting, he added.Attempts to send a cargo plane with 16 tons of urgently needed medical supplies to the rebel-held capital Sana'a were ongoing, Red Cross spokeswoman Sitara Jabeen said."There are still logistical issues with the plane - mainly the difficulty of flying into a conflict zone," she told dpa.