Yemen crisis: Rebels push into central Aden
Fresh fighting has been reported in the southern Yemeni city of Aden between Houthi rebels and militiamen loyal to President Abdrabbuh Mansour Hadi.
Several houses in the central Crater district were set on fire after being hit by rockets as the rebels advanced, residents told the Reuters news agency.
Warplanes of the Saudi-led coalition, which backs the government, meanwhile bombed rebel targets to the north.
Iran is also reported to have sent navy vessels to the Gulf of Aden.
Navy commander Rear Adm Habibollah Sayyari was quoted as saying by state-run Press TV that a destroyer and another ship would be deployed off Yemen as part of an anti-piracy campaign "safeguarding naval routes for vessels in the region".
Tehran has denied Saudi accusations that it is providing military and financial assistance to the Houthis, who adhere to a branch of Shia Islam known as Zaidism.
Aid shipment arrives
The campaign by Sunni-ruled Saudi Arabia, which seeks to "defend the legitimate government" of Mr Hadi, has so far failed to stop the Houthis' assault on Aden.
The president took refuge in the second city in February after fleeing the capital, Sanaa, where he had been held under house arrest by the rebels. When the Houthis reached the outskirts of Aden on 25 March, he left the country.
On Wednesday, dozens of rebel fighters and allied troops reportedly pushed into the district of Crater, near the city's port. Residents told Reuters that the rebels were backed by a tank and two armoured vehicles.
They also said that from loudspeakers on Sunni mosques a call had rung out for local people to "rise for jihad" against the attackers.
Reuters also reported three explosions in northern areas of Aden, which residents said were strikes on rebel weapons depots.
At the scene: Orla Guerin, BBC News, Sanaa
Little is going in or out of war-torn Sanaa these days, apart from Air India.
The airline has been running rescue missions for its own nationals and for foreigners from more than 26 countries. We flew with them from Djibouti to Yemen - a four-hour round trip.
Once on the ground, there were signs of the escalating conflict. We taxied past aircraft hangers targeted in the recent air strikes. From the runway we could see smoke rising in the distance beyond the airport.
A lone Houthi rebel was waiting on the tarmac. After asking a few questions he posed for a group photo with the Air India crew.
Within minutes passengers were rushing onto the plane. Young and old were crammed in, some sitting six to a row. The aircraft had a capacity of 182, but took off with about 240 on board.
Air India hoped to leave no-one behind but around 200 people remained in the terminal. One man on board told us he had to pay a $1,000 (£669) bribe to the Houthis to get his family through the airport.
After days of street fighting, conditions for civilians in Aden have been described as catastrophic.
But a boat carrying 1.7 tonnes of medical aid for Medecins Sans Frontieres (MSF) has managed to dock in Aden for the first time since the air campaign began.
The International Committee of the Red Cross (ICRC) also said it had now received clearance from all warring parties in Yemen to deliver aid supplies.
A ship, carrying medical supplies and a surgical team, is expected to arrive in Aden later on Wednesday, and two planes will fly in further medical supplies as well as generators, tents and equipment for repairing damaged water systems.
An ICRC spokeswoman called the development "very good news" but said big challenges lay ahead, particularly in the distribution of aid to hospitals already damaged by the conflict.
The World Health Organisation has said that at least 560 people, including 76 children, were killed in Yemen between 19 March and 4 April. Another 1,700 people were wounded and 100,000 fled their homes.
Meanwhile, the US has said it is stepping up its support of the coalition.
"We have expedited weapons deliveries, we have increased our intelligence sharing, and we have established a joint co-ordination and planning cell in the Saudi operations centre," Deputy Secretary of State Antony Blinken told reporters in Riyadh after meeting Mr Hadi and Saudi officials.
Mr Blinken accused the rebels, and allied security forces loyal to former President Ali Abdullah Saleh, of wrecking Yemen's economy and institutions, and creating instability that jihadist militants would seek to exploit.