One of the EU's most radical weapons against climate change, ultimately capable of delivering some of the emissions cuts that have eluded the Kyoto process, is under new attack.

The law makes all airlines flying in and out of the European Union pay for their carbon using the EU's Emissions Trading Scheme (ETS). Its extraterritorial reach has stirred international outrage.

To try to force the EU to change course, U.S. politicians have proposed blocking legislation, which could be used to prohibit airlines from complying with the EU law. the U.S. Congress will hear testimony to assess the draft proposals.

India and China have meanwhile warned of retaliation that could escalate into a trade war and their airlines have failed to comply with preliminary EU deadlines ahead of the first bill due next April.

Russia, significant because many international flights cross its airspace, has also said the EU needs to be flexible.

"I understand it might be difficult for the EU to backtrack on its decision. Some people compare the EU to a crocodile, not because of its teeth, but because of its inability to move backwards," Ambassador Vladimir Chizhov, Russia's permanent representative to the EU, said just before EU-Russia talks in St Petersburg at the start of this week.

EU Climate Commissioner Connie Hedegaard, has said the only reason for the EU to modify its law would be if the U.N.'s International Civil Aviation Organization (ICAO) forges a global scheme to curb rising airline emissions.

As tensions over the law have flared, efforts have intensified at ICAO, which later this month is expected to debate progress on an internationally agreed alternative to the EU plan.

Typically ICAO has moved at a glacial pace and few expect a swift outcome.

Gabriel Sanchez, adjunct professor of law at the International Aviation Law Institute at Chicago's DePaul University, is among the doubters.

"It's my contention that the only feasible way to build up an aviation carbon market is through incremental, country-by-country cooperation," he said.

"If the U.S. and EU, for example, could come to some sort of an agreement, it would have a powerful demonstration effect, which might lead other countries to join in over time."

In the absence of an ICAO plan, Sanchez's said the EU would only retreat if the threats against it materialised.

"The EU won't back down until the U.S. takes some serious action, such as imposing retaliatory charges on EU airlines or some other form of trade sanctions," he said.

A real show-down is likely to be months away.

The full force of the EU law, which began to take effect on Jan. 1, strikes in next April when the emissions bills are due.

Meanwhile, this year's U.S. presidential election is expected to defer difficult decisions - such as a presidential signature on the blocking legislation that could leave airlines facing the impossible choice of breaking either U.S. or EU law.

Trade War Over a Cup of Coffee?

Even when the payment demands arrive, the cost is moderate, the Commission says, with some 85 percent of the permits needed initially handed out free.

For a flight from Beijing to Frankfurt, the Commission cites figures showing that the scheme would add around 2 euros ($2.47) per passenger. Many airlines have already added them to fares.

"Are you really going to start a trade war for less than the cost of a cup of coffee at the airport?" asked Commission spokesman Isaac Valero-Ladron.

Penalties of non-compliance are much higher at 100 euros for each tonne of carbon not purchased. Repeat breaches of the law, validated by the EU's highest court, could lead to a ban, although the EU has said that would be a last resort.

A factor that could calm U.S. opposition, analysts say, is one that has incensed China and India, namely that the EU law makes all emitters pay equally.

China and India, the world's biggest and third biggest emitters (the United States ranks second), have argued, within the Kyoto framework, that the developed world is mo