Tensions over global warming between Downing Street and the White House will increase today with a warning by a senior British minister that the US should sign up to UN targets for reducing climate change.

The Environment Secretary, David Miliband, will use a speech in George Bush's backyard to warn that the US President's plan for delaying a deal to the end of 2008 misses the point that urgent action is needed now. Speaking at a conference in Washington on global warming, the Environment Secretary, who is expected to get a senior appointment in Gordon Brown's cabinet, will say that the case for urgent action is "unambiguous".

Mr Miliband told the Commons yesterday: "I think it's not helpful for anything that muddies the waters about the primacy of the UN process. But I don't think that is what the President said." He added that Mr Bush's acceptance of the need for voluntary targets on cutting CO2 emissions had removed a roadblock to discussions this week at the G8, but it was not the same as "driving all the way down the road".

Mr Miliband will make it clear in his speech today that Britain believes the President's speech marked only the first step and Mr Bush should follow it up by committing the US to accepting global targets at the UN climate change conference in Bali.

He will say: "President Bush's call for a long-term global goal for emissions reductions and his commitment to technology transfer is significant.

"It represents the first, not the last, step towards a global agreement on emissions reductions. It is vital we continue to make progress at this week's meeting of the G8 in preparation for the meeting of the UNFCC in Bali."

Mr Miliband is keen to avoid appearing to lecture the Bush administration, but he will include a pointed reference in his speech to the political advantages of signing up to global targets . He will remind the Republicans who have been reluctant to do a deal at the G8 that climate change will be a key issue in the next presidential elections for the first time.

Tony Blair was careful to sound positive about President Bush's speech on Friday, saying it signalled a shift by the US to accept global targets, but Mr Bush's reluctance to accept the UN process drew concern from Downing Street.

The Prime Minister's official spokesman said Britain wanted the US effort to complement work in the UN. He said: "We do believe any process should be complementary to the UN conference in Bali and should fit in with that of course because that is the recognised process."

He insisted that President Bush's initiative on climate change was significant because it acknowledged for the first time the need for a global limit on emissions. The spokesman, however, refused to drawn on Britain's hopes for the G8 summit, saying: "Our position is we want to push matters as far was we can. It's for the G8 to decide how far we get. Let's see what happens at the end of the week."

Giving evidence to the Commons Select Committee on Environmental Audit, Mr Miliband said that the US has a "huge amount to gain from putting itself at the head of this and that is increasingly recognised by US states, US cities and US business. I hope to learn more about how evangelical groups, how politicians, how the states, all of whom I am meeting, see that."