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UPDATE 1-U.N. chief says world will lose brake on nuclear war with end of INF treaty

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UNITED NATIONS, Aug 1 (Reuters) - U.N. Secretary-General Antonio Guterres is concerned by rising tensions between nuclear-armed states, warning "the world will lose an invaluable brake on nuclear war" with the expiration of the Intermediate-range Nuclear Forces Treaty (INF) on Friday.

"This will likely heighten, not reduce, the threat posed by ballistic missiles," Guterres told reporters. "Regardless of what transpires, the parties should avoid destabilizing developments and urgently seek agreement on a new common path for international arms control."

Barring a last-minute decision by Russia to destroy a new medium-range missile that NATO says violates the INF, the United States is set to pull out of the accord on Aug. 2, arguing that it needs to develop its own warheads to deter Moscow.

Moscow says it is fully compliant with the treaty, negotiated by former U.S. President Ronald Reagan and Soviet leader Mikhail Gorbachev, which eliminated the medium-range missile arsenals of the world's two biggest nuclear powers.

The breakdown of the treaty, the latest in a growing list of East-West tensions, is of grave concern because medium-range rockets would allow Russia to launch a nuclear attack on Europe at very short notice, Western experts and officials say.

"I strongly encourage the United States and the Russian Federation to extend the so-called New Start agreement to provide stability and the time to negotiate future arms control measures," Guterres said.

The 2011 New START treaty, a U.S.-Russia arms control pact which limits deployed strategic nuclear weapons, is set to expire in February 2021 but can be extended for five years if both sides agree.

Guterres also said he was "troubled by growing friction" between the economies of the United States and China. He warned of the possible "emergence of two competing blocs – each with their own dominant currency, trade and financial rules, their own internet and artificial intelligence strategy, and their own contradictory geopolitical and military views."

The United States and China have levied billions of dollars of tariffs on each other's goods in a year-long trade war, disrupting global supply chains and roiling financial markets. (Reporting by Michelle Nichols Editing by Alistair Bell)