The United States said relief channels were belatedly opening up in the typhoon-ravaged Philippines on Thursday as the UN admitted it had not acted quickly enough to help survivors.

The aircraft carrier USS George Washington arrived off the Philippines to support a massive international aid effort, one of eight American ships in the region.

Britain announced that it was sending its largest naval ship, the helicopter carrier HMS Illustrious, and that British government aid now exceeded 20 million pounds ($32 million, 24 million euros).

President Barack Obama urged Americans to dig deep and other countries upped their aid. The UN's humanitarian chief Valerie Amos said the scale of the disaster and logistical challenges meant that six days after Super Typhoon Haiyan struck, some places remained without help.

"I very much hope that over the next 48 hours that that will change significantly," she told reporters in Manila. "I do feel that we have let people down."

UN leader Ban Ki-moon, currently in Latvia, added: "Now is the time for the international community to stand with the people of the Philippines.

"They are showing great courage and resilience, but they need our help and they need it now."

After criticism at home and abroad of China's initial offer of a $100,000 cash donation, Beijing will provide an additional 10 million yuan ($1.6 million) for relief efforts in the form of blankets, tents and other materials.

Transport planes, helicopters, ships and medics are in operation or coming from an array of countries in the Asia-Pacific and Europe, with Australia now raising its total aid contribution to Aus$30 million (US$28 million).

On the ground in the shattered city of Tacloban and around the central Philippines, survivors are pleading for food, water, clothes and medicines -- and security to protect them from mobs pilfering what little aid is getting through.

US officials said the aid operation was slowly overcoming the challenges posed by shattered ports, roads and communication infrastructure.

Washington has committed $20 million.

Its giant carrier, carrying 5,000 sailors diverted from shore leave in Hong Kong, has the ability to desalinate large volumes of water.

"The friendship between our two countries runs deep, and when our friends are in trouble, America helps," Obama said in a statement.

One US official said relief workers were now better able to distribute aid out of Tacloban airport, and the opening of a land route had given a significant boost by connecting to a sea port.

Other recent aid moves include:

-- The United States is doubling the number of MV-22 Ospreys -- rotor planes that can take off and land like helicopters -- to eight, joining a team of Marines who are equipped with amphibious vehicles and 12 cargo planes delivering food, water and other essentials.

-- Britain has sent a Royal Air Force C-17 transport aircraft. It is also dispatching a 12-member medical team from the state-run National Health Service. Three chartered flights with aid equipment have already arrived. A public appeal has raised 23 million pounds in the first 48 hours.

-- The United Nations has launched a flash appeal for $301 million to help with the immediate aftermath of the disaster, which it has said could have already cost 10,000 lives -- though the Philippine government says the toll will be lower.

-- India has sent an air force C-130 aircraft carrying medicines, hygiene goods, chemicals, tents, blankets, tarpaulins and ready-to-eat meals.

-- The International Labour Organization is working on emergency employment as part of the UN's appeal. The ILO estimates that three million people have lost their livelihoods, at least temporarily.

-- Australia dispatched another C-130 Hercules transport aircraft with a fourth plane on standby, while the amphibious landing vessel HMAS Tobruk is being diverted to the Philippines.

New Zealand is also sending a C-130 Hercules carrying emergency supplies.

-- Sweden has sent three aircraft carrying aid and base camps to serve as the UN's disaster relief hub, as well as 21 European experts in fields including communications and water purification.

-- Norway will increase its aid by 20 million crowns to 65 million crowns ($10.5 million).

-- South Korea has pledged $800,000 of emergency relief; two C-130s carrying aid have left for Tacloban and are due to remain there for 10 days.

-- Japan said it was ready to send as many as 1,000 troops from its Self-Defense Forces, along with three naval ships and an unspecified number of aircraft. That would be the Japanese military's single largest relief operation abroad since World War II. It also pledged $10 million for emergency shelters and other help, through aid organisations.

-- France has delivered a shipment of 10 tonnes of emergency equipment including tents and tarpaulins, while a Dutch air force DC-10 carried 29 tonnes of canvas, water sterilising equipment, medicines and solar lighting. Teams of medical experts have been sent from Belgium and Spain.

-- Malaysia is sending aid flights and military medics, while Indonesian planes are delivering supplies including water filters. Thailand is adding to the regional relief effort with some $2.7 million in aid and an unspecified number of C-130s to bring medical supplies. Vietnam has pledged $100,000 plus $50,000 from the Vietnamese Red Cross.

-- Kuwait, which like other Gulf countries is home to a large number of Filipino workers, released $10 million in urgent relief aid.

-- Charities and companies worldwide are also donating funds. In one of the biggest non-governmental operations, Medecins Sans Frontieres (Doctors Without Borders) is sending eight planeloads of aid including vaccination kits, tents and sanitation equipment.