Russia said on Monday it would unblock the delivery of sophisticated S-300 missile defense systems to Iran in a move that alarmed Israel and could see Russia and the United States engage in a proxy arms race in the Middle East.

The sale of even more advanced S-400 air defenses to China was confirmed on the same day.

Secretary of State John Kerry objected to the planned delivery of the defense system to Iran in a phone call to his Russian counterpart, Sergei Lavrov. A White House spokesman said the decision could endanger plans to ultimately lift economic sanctions on Iran as part of a proposed nuclear deal.

However, Lavrov argued in a televised statement that the preliminary agreement world powers struck with Iran earlier this month to prevent the country from developing atomic weapons meant the ban was no longer justified.

It is difficult not to see Russia’s decision within the context of deteriorating East-West relations.

A year after the two former rival superpowers announced a “reset” in their relationship, Russia suspended the S-300 deal in 2010 in what Lavrov described on Monday as a “good faith” gesture meant to draw Iran into negotiations about its nuclear program which Israel and the West fear could see it producing a bomb.

Russia lifted the ban a year after the European Union and the United States sanctioned individuals with close links to President Vladimir Putin’s regime in retaliation for its annexation of the Crimean Peninsula from Ukraine. The sanctions have pushed Russia’s economy into recession. Russia embargoed European fruit and vegetable exports in return.

Defense analysts told the Financial Times it was uncertain Russia would go ahead with the delivery. Ruslan Pukhov, the director of the Moscow-based Center for Analysis of Strategies and Technologies, said Russia would “simply not allow” Iran to get the S-300s for fear of upsetting its improving relations with Israel.

Brenda Shaffer, a professor at Georgetown University, said Russia has a history of “dangling” the prospect of selling the S-300s whenever it wants concessions from the West.

At least six times in the past, they have threatened to do this but they have never actually gone through with it.

Most recently in 2013, Iran announced that Russia had agreed to deliver the S-300s yet the missile systems never arrived.

The S-300s have a range of up to 200 kilometers and can track down and strike multiple targets simultaneously, making them one of the world’s most potent air defenses. Their deployment could deter Israel from carrying out unilateral airstrikes against Iran’s nuclear facilities.

The delivery could also compel the Americans to bolster their military support for Arab states that see Iran as their rival for hegemony in the region. Saudi Arabia and the smaller monarchies in the Persian Gulf are highly dependent on American military aid. So is Egypt.

Earlier this month, the United States unfroze $1.3 billion in yearly military assistance to Egypt and promised to deliver new Abrams tanks, F-16 fighter jets and Harpoon missiles after the country had opened talks with Russia to start what strongman Abdul Fatah Sisi described as “a new era of constructive, fruitful cooperation on the military level.”

Russia, on the other hand, still has ties with Syrian dictator Bashar al-Assad, an Iranian ally, while other Arab countries and the United States support the largely Sunni uprising against him. In late 2014, Syrian rebels revealed equipment and photographs they said proved Russian military intelligence or special forces personnel had been jointly operating a military base near the Golan Heights with the Syrian army.

If Russia goes through with the Iran deal — this time — it could see a resumption of the arms race by proxy with the United States that also played out in the Middle East during the Cold War.

Before 1979, when Egypt and Israel made peace and religious fanatics overthrew the regime of the shah in Iran, the Soviet Union supported the Arab states with weapons while the United States backed Israel and Iran. Since the American realignment in favor of the Arabs, Russian influence in the region has waned.