By Tim Hirsch
BBC Environment Correspondent, Montreal
Canada's prime minister has urged the US to "listen to its conscience" and take further steps to reduce emissions linked to global warming.
Martin made a pointed appeal
Paul Martin was speaking at the UN climate change conference in Montreal, where talks on long-term strategies are reaching a critical stage.
It is not only the United States which has come under fire.
A British government minister has accused Saudi Arabia of using "outrageous" tactics to block progress.
'In this together'
Mr Martin, who is in the second week of his own campaign for re-election, formally opened the ministerial section of the conference on Wednesday.
His speech was clearly aimed at the United States, although he did not mention the country by name.
He told delegates from around 190 countries that "climate change is a global challenge that demands a global response, yet there are nations that resist...
"Well, it is our problem to solve. We are in this together."
At a news conference after the speech, Mr Martin referred explicitly to Canada's neighbours to the south, saying: "To the reticent nations, including the United States, I would say this: there is such a thing as a global conscience, and now is the time to listen to it."
One of the main controversies at this conference is an attempt by the Canadian Environment Minister, Stephane Dion, who is chairing the talks, to launch a two-year process of discussions outside the jurisdiction of the Kyoto agreement, which the US has rejected.
The aim is to set up a "parallel track" to discuss various options for taking forward international action, which would enable both the Americans and developing countries to participate without committing themselves to specific targets on cutting or limiting emissions.
The theory is that this would make it easier for those countries signed up to Kyoto to commit themselves to stronger legally-binding emission cuts when the current targets expire in 2012.
On the face of it, the Canadian proposal is so bland as to be meaningless, calling for "discussions to explore and analyse approaches for long-term co-operative action".
The wording has in fact been carefully chosen to mirror language used in previous agreements at Gleneagles under the UK presidency of the Group of Eight (G8) leading economies, and at the UN World Summit in New York.
Both of those were signed up to by the United States.
Even so, this proposal has so far been rejected by the US, which is wary of entering any process which might be seen as the first stage of a new Kyoto-style agreement.
Asked about the Canadian proposal, the head of the US delegation in Montreal, Paula Dobriansky, said "the best approach is one that takes into account diversified approaches and differing opinions. One size does not fit all."
This attitude is causing exasperation amongst negotiators from the European Union, who feel that it is a step backwards from commitments made by President George W Bush during this year's British presidency of the G8.
But UK Environment Minister Elliott Morley reserved his strongest criticism for the tactics being used by Saudi Arabia. The Saudis are insisting on a procedure which could delay for years the system of policing the Kyoto agreement - and claiming that oil-producing states should receive compensation from industrialised countries for the lost revenues they could suffer if the world moves towards cleaner fuels.
"They are the villains of the piece," Mr Morley told reporters in Montreal.
He described the Saudi tactics as "outrageous", claiming that they were blocking every discussion and exerting a negative influence on the conference.