US President George W Bush is set to pledge $674m (£350m) in aid for Africa as part of a joint initiative with visiting UK Prime Minister Tony Blair.
Tony Blair's trip is part of preparations for the G8 summit
Mr Blair is in Washington to press for the president's support for his plans to get Africa back on its feet.
Mr Bush has already rejected several key parts of Britain's relief plan - but Mr Blair insisted there had been "significant progress" on a deal.
Aid groups criticised Mr Bush's cash pledge as a mere "drop in the ocean".
Jonathan Glennie of Christian Aid said: "If this is President Bush's only response to the crisis in Africa, we think he cannot be serious about alleviating poverty."
The $674m is destined largely for Ethiopia and Eritrea, countries threatened by famine, and for humanitarian needs in other African countries.
This money is part of the US aid budget that had already been announced but had not yet been allocated to a country.
Britain is also expected to contribute money to the cause.
The US has also set aside $1.4bn (£767m) requested by the United Nations to address emergency needs.
Nevertheless, Mr Blair recognises that he will not get US support for crucial parts of his three-pronged attack on poverty in Africa - a package of debt relief, increased aid and fairer trade that is the centrepiece of his chairmanship of the G8.
In an interview for the Financial Times newspaper on Tuesday, Mr Blair acknowledged: "There are certain things we know they are not going to do, that we are not asking them to do."
Mr Bush has already opposed UK treasury chief Gordon Brown's plan to use an international finance facility (IFF) to fund vaccinations, funded by borrowing on the bond market, saying he cannot commit the US to future debt repayments.
The US has also refused to agree to give 0.7% of its national income in international aid, a long-term commitment Mr Blair wants from all G8 countries.
"We are not asking them to sign up to the IFF or 0.7% in aid. They are not going to do that and they've made that clear right from the very beginning," Mr Blair said.
Speaking later in Washington, Mr Blair insisted: "We are a significant way toward a deal" on debt relief.
He also said that he would be pushing Mr Bush for more.
"It is important we deal with the situation in Ethiopia and Eritrea, but obviously, there's a lot more that needs to be dealt with...the [Bush] administration itself has made clear that this is not the only commitment."
Washington's reluctance to join Mr Blair's crusade against poverty has angered aid groups and development experts.
"The US is not pulling its weight right now," said Professor Jeffrey Sachs of Columbia University's Earth Institute.
He said there was "a great myth in the US" that aid was ineffective because of inefficiency and corruption.
"It's a nonsense. Aid works - the problem is it's on such a small scale that it's not commensurate with the challenge," he told BBC Radio 4's Today programme.
Oxfam said Mr Blair "must not cave in to US pressure and water down proposals on aid, trade and debt... to lower the ambition at this critical stage, would be seen by many as a betrayal of Africa".
Mr Blair's spokesman acknowledged that battling poverty was "about more than throwing money at the problem".
He played down expectations for the meeting with the US president, saying the visit was part of the preparation for the summit at Gleneagles, Scotland, "not Gleneagles itself".
Mr Bush and Mr Blair are also expected to make a joint announcement on climate change - another British priority for Gleneagles.
The two countries also have different approaches to that issue.
The US favours a technology-based solution to global warming over targets to curb greenhouse gases.
Mr Blair told the Financial Times he was not asking the US "to reverse [its] position on Kyoto. There's no way the Americans are going to do that."
But he said he was still hopeful of a breakthrough.