Washington is leading calls for UN sanctions against Iran for failing to suspend uranium enrichment, and warnings to North Korea over its plans to explode a nuclear device.

But key Iranian ally Russia is reluctant to impose sanctions on Tehran, while China, worried about the fragile regime across its border, cautions against provocative actions against Pyongyang.

"These countries are part of the 'near abroad' in a way they are not for the United States," said William Hopkinson, from the London think tank Royal United Services Institute, referring to China's relationship with North Korea and Russia's with Iran.

Hopkinson said that being neighbors meant that "geographic and strategic" as well as economic interests had to be taken into account.

He added that Russia tended to follow China on the North Korean issue as Beijing was the direct neighbor, while China follows Russia's lead over Iran.

"In the case of the Chinese, they don't want North Korea to have nuclear weapons but they don't want to take steps either that will destabilize North Korea and give them problems on their borders," said Gary Samore, a non-proliferation expert at the Council on Foreign Relations think tank in New York.

French analyst Francois Heisbourg said: "China has a real fear that North Korea would implode. Regime change in North Korea is nothing China is happy with" as it could lead to "millions of refugees flooding into Manchuria" on China's border with North Korea.

Russia, which has a lucrative contract to build Iran's first nuclear reactor and must deal with Iranian influence in central Asia, "does not want Tehran to have nuclear weapons but doesn't want to take steps that will damage its relationship" with the Islamic Republic, added Samore.

This leaves the United States to set a tough line as Iran and North Korea's nuclear programs come under the spotlight.

Iran is defying the threat of United Nations sanctions by refusing to suspend key uranium enrichment.

While North Korea said this week that it was ready to test a nuclear device.

But Heisbourg, a former French foreign ministry non-proliferation expert, cautioned it would be a mistake to "overestimate" Russia's reluctance to support punitive measures against Iran.

He said Tehran has humiliated Moscow by rebuffing Russian efforts to strike a compromise on the thorny issue of enrichment, the process which makes fuel for civilian nuclear reactors but also the raw material for atom bombs.

Washington, which has long been in a standoff with North Korea over its nuclear weapons ambitions, directly warned Pyongyang on Wednesday that it would pay a price if it goes ahead with the nuclear test.

Yet on Thursday, US Ambassador to the UN John Bolton spoke of "division" within the council, suggesting "North Korea's protectors" -- implying Moscow and Beijing -- had opposed his call for a tough line against Pyongyang.

China's UN envoy Wang Guangya took exception.

"We are all concerned about the North Korean announcement. ... No one is going to protect them," he said.

Wang did however urge "less mistrust" between Washington and Pyongyang.

Samore said the United States seemed more willing to compromise when it had bilateral relations of its own on the line.

It recently struck a deal to help out India's nuclear program, even though India has defied the international community by having nuclear weapons and refusing to sign the Non-Proliferation Treaty.

This is a case "where the United States has non-proliferation interests but other political and economic interests as well," Samore said.

US "non-proliferation (policy in India) is not pure. It's adulterated with other strategic interests. That's not true in Iran and North Korea" where Washington "sees these countries as enemies," said Samore