Republican White House candidate John McCain Monday veered sharply away from President George W. Bush on climate change, saying he would not "shirk" from the need for US global leadership.

The Arizona senator proposed a mandatory cap-and-trade system to cut greenhouse gas emissions, and implicitly rebuked Bush for his skepticism on anti-climate change efforts.

The initiative also signaled that McCain plans to challenge Democrat Barack Obama for independent voters, should he face him in November's presidential election, on an issue of rising importance in US politics.

"I will not shirk the mantle of leadership that the United States bears," McCain said in a speech at a wind power plant in the western state of Oregon.

"I will not permit eight long years to pass without serious action on serious challenges," he said, in a clear rebuke to the Bush administration.

McCain also pledged to play a lead role in negotiations for an agreement to come into force after the Kyoto treaty's commitments on emissions cuts, which the United States refused to ratify, expire in 2012.

"I will not accept the same dead-end of failed diplomacy that claimed Kyoto," McCain said.

"The United States will lead and will lead with a different approach -- an approach that speaks to the interests and obligations of every nation."

Bush objected to Kyoto because it did not apply binding greenhouse gas targets on fast-growing China and India.

McCain proposed a cap-and-trade system, which sets a limit of total greenhouse gas emissions but allows companies to sell unused emission credits to other firms that have exceeded their quotas.

His plan would seek to return emissions to 2005 levels by 2012, and to 1990 levels by 2020. It foresees a reduction of 60 percent below 1990 levels by 2050.

Plans to address climate change by the two Democrats still in the White House race would go further than McCain, and Obama accused the Republican of double-speak on the issue.

"It is truly breathtaking for John McCain to talk about combating climate change while voting against virtually every recent effort to actually invest in clean energy," he said in a statement.

The Illinois senator flagged up McCain's opposition to a 2005 energy bill, which contained incentives for alternative energy such as wind power but was also criticized for its generosity to the oil industry.

"In stark contrast, I've called for a national standard to ensure that we're using more renewable energy, an expansion of our green energy sector that would create millions of green jobs, and a bipartisan plan to double our fuel efficiency standards," Obama said.

"That is why the American people will have a clear choice in November when I am the nominee -- between a candidate who opposes real solutions to our energy crisis, and leadership that will solve it once and for all."

Clinton accused McCain of taking "halfway measures" to address climate change.

"While Senator McCain's proposals may be an improvement on President Bush's, that's not saying much," she said in a statement.

Clinton and Obama both advocate a cap-and-trade system to reduce greenhouse gas emissions by 80 percent below 1990 levels by 2050.

McCain's stance sets him apart from a large chunk of his party's conservative base, which remains skeptical about the science on climate change.

Bush last month laid out a blueprint to curtail the growth of US greenhouse gas emissions from 2025, but critics said it would do little to combat climate change.

The White House has also rejected a bipartisan effort in Congress, which could come up for debate within weeks on a cap-and-trade plan, saying it is too complicated and would be bad for the US economy.