The BBC's Jon Donnison reports from a street
destroyed by "a wall of water"
The US has deployed an aircraft carrier and navy ships, while the UK is sending a naval destroyer.
At least 10,000 people are feared to have been killed and thousands of survivors desperately require aid - but reports say little is getting through.
Philippine President Benigno Aquino has declared a state of national calamity.
In a statement, he said the two worst affected provinces, Leyte and Samar, had suffered massive destruction and loss of life.
He authorised the release of emergency relief funds and deployed troops to affected areas.
A huge international relief effort is also under way, but journalists and rescue workers at the scene say reaching areas affected by the storm is difficult.
Bernard Kerblat, who is overseeing the UNHCR response to the crisis, said some aircraft had landed in Cebu but distributing aid was difficult because of bad weather and damaged infrastructure.
"The rain is further complicating the effort for light vehicles, including trucks, to penetrate in areas wherever there's still a bridge left intact.
"The other bad news is that within the next 72 hours, we should see the arrival of yet another typhoon."
Large parts of the Philippines have been
devastated by Typhoon Haiyan
In Tacloban, Leyte, hundreds have gathered at the airport
for supplies or to try and leave the city
Thousands of survivors desperately need aid,
including food and water
The Philippine president has declared a state
of national calamity
'Heartbreaking'
The National Disaster Risk Reduction and Management Council(NDRRMC) said 1,774 people had been reported dead, 2,487 were reported injured and 82 were missing.
The death toll is expected to rise significantly in coming days.
More than 580,000 people had been displaced and 41,000 houses had been damaged, the NDRRMC added.
UN Secretary General Ban Ki-moon described images of the impact of the storm as "heartbreaking".
The UN would launch a large-scale humanitarian plan and allocate $25m (£15.5m) "to fund critical relief efforts", he said.
"Many thousands of people are reported to have died and almost 10 million people have been affected... Let us all show our solidarity with the people of the Philippines at this time of need," he added.
BBC correspondent Rupert Wingfield-Hayes reports on the "stinking black water" left behind
On Tuesday, heavy thunderstorms struck Tacloban, one of the areas worst-hit by the typhoon. Correspondents say driving rain has added to the misery of tens of thousands of people living amid the wreckage of their homes.
One of the most powerful storms on record to make landfall, Haiyan - named "Yolanda" by Philippine authorities - struck the coastal provinces of Leyte and Samar on Friday.
It then headed west, sweeping through six central Philippine islands.
Air Force Capt Antonio Tamayo told AP news agency the scene in Tacloban, one of the worst-hit areas, was "overwhelming".
"We need more medicine. We cannot give anti-tetanus vaccine shots because we have none.''
Officials said looting was widespread and order was proving difficult to enforce. Correspondents say many ordinary people are simply scavenging for the food and water needed to survive.
The government says it has deployed armoured vehicles to Tacloban to deter looters.
"We are circulating [the vehicles] in the city to show the people, especially those with bad intentions, that the authorities have returned," Interior Secretary Mar Roxas told DZMM radio.
Alison Wallace, chief executive of disaster relief charity Shelterbox, told the BBC that delivering aid safely would be a major concern.
"Security is going to be a big part of the operation," she said.
"We have to make sure that when the aid is delivered that we don't actually make the situation worse by creating difficult scenes for people - the aid has to be delivered fairly [and] with as much safety as possible."