Here are some frequently asked questions about the pact.

WHAT IS THE KYOTO PROTOCOL?

It is a pact agreed by governments at a 1997 U.N. conference in Kyoto, Japan, to reduce the amount of greenhouse gases emitted by developed countries by 5.2 percent of 1990 levels during the five-year period 2008-2012. A total of 122 nations have ratified the pact or acceded to it, according to U.N. data.

IS IT THE FIRST AGREEMENT OF ITS KIND?

Governments originally agreed to tackle climate change at an "Earth Summit" in Rio de Janeiro in 1992. At that meeting, leaders created the U.N. Framework Convention on Climate Change (UNFCCC) which set a non-binding goal of stabilizing emissions at 1990 levels by 2000, a goal not met overall. The Kyoto protocol is the follow-up to that and is the first legally binding global agreement to cut greenhouse gases.

SO IT'S LEGALLY BINDING?

It is binding once it has been ratified by 55 percent of the signatories which must between them represent 55 percent of developed countries' carbon dioxide emissions. Kyoto has surpassed the requirement of signatories but has so far only received pledges from nations representing 44 percent of total emissions.

Russia holds the key to Kyoto's success or failure with its 17 percent share of emissions. President Bush pulled out in 2001, arguing that Kyoto was too expensive and unfairly excludes developing nations. The United States is the biggest polluter with a 36 percent share.

HOW WILL IT BE ENFORCED?

Under a 2001 deal made by environment ministers in Bonn, Germany, if countries emit more gases than allowed under their targets at the end of 2012, they will be required to make the cuts, and 30 percent more, in the second commitment period which is due to start in 2013. They rejected the idea of a financial penalty.

DOES EVERY COUNTRY HAVE TO REDUCE EMISSIONS BY 5.2 PERCENT?

No, only 39 countries - relatively developed ones - have target levels for the 2008-12 period, adhering to the principle established under the UNFCCC that richer countries should take the lead. Each country negotiated different targets, with Russia aiming for stabilization at 1990 levels and the European Union trying for an eight percent cut.

HOW ARE THEY DOING SO FAR?

Most countries are lagging their targets under Kyoto. The European Commission said in December, for instance, that 13 of the then 15 members of the European Union were likely to overshoot the target. Only Britain and Sweden were on track.

WHAT ARE THESE "GREENHOUSE GASES?"

Greenhouse gases are gases that trap heat in the earth's atmosphere. The main one is carbon dioxide (CO2), most of which comes from burning fuel. The protocol also covers methane (CH4), much of which comes from agriculture and waste dumps, and nitrous oxide (N2O), mostly a result of fertilizer use.

Three industrial gases used in various applications, such as refrigerants, heat conductors and insulators, are also included - they are hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulfur hexafluoride (SF6).

WHAT HAPPENS TO COUNTRIES THAT MISS THE TARGET?

The protocol provides for "flexible mechanisms" - ways for countries to reach their targets without actually reducing emissions at home.

These include emissions trading - where one country buys the right to emit from another country which has already reduced its emissions sufficiently and has "spare" emissions reductions.

Another is the "clean development mechanism" where developed countries can earn credits to offset against their targets by funding clean technologies, such as solar power, in poorer countries.

Countries can also claim credits for planting trees in the Third World that soak up CO2 - so-called carbon "sinks."

While Iran is providing access, it has balked at the IAEA's demand that it freeze all activities on uranium enrichment.

Friday a leading Iranian cleric said his country would never be bullied into giving up its nuclear program but denied an weapons' ambitions.

"Iran will never yield to international pressure to abandon its home-grown nuclear technology," Ayatollah Ahmad Jannati, who heads Iran's hardline Guardian Council - a powerful, unelected supervisory body - told worshippers at prayers in Tehran.

"Americans should know that it is just impossible. You will take this wish to the grave," he added. "We have no intention of pursuing nuclear weapons."

Washington says Tehran is developing weapons under cover of a civilian atomic energy program and wants it reported to the Security Council. Tehran vehemently denies the accusation.

Iranian officials were not available for comment, though a diplomat close to the negotiations said Iran had agreed in principle to allow IAEA inspectors to visit all sites.

David Albright, a former U.N. weapons inspector and head of the Institute for Science and International Security, said Parchin was a "huge site dedicated to the research, development, and production of ammunition, rockets, and high explosives."

A senior U.S. official told Reuters last month that Iran had refused to allow the U.N. into Parchin, which he said was a strong indication that it was a nuclear weapons site.

He also accused the IAEA of suppressing information on Parchin in its latest Iran report - a charge the agency denied.