A senior Russian diplomat on Wednesday angrily rejected U.S. warnings against striking an oil-for-goods contract with Iran, saying that Moscow wouldn't be intimidated by threats.

Deputy Foreign Minister Sergey Ryabkov said in remarks carried by the state RIA Novosti news agency that an increase in Russian-Iranian trade is a "natural process that doesn't involve any elements of political or economic challenge to anyone."

Russian business daily Kommersant has reported that Moscow plans to buy 500,000 barrels of Iranian oil a day, a deal that would shatter an export limit defined by an interim nuclear agreement world powers and Iran reached last year.

Iran has agreed to temporarily limit its atomic work, which the West fears could be a cover for developing nuclear weapons, in return for some sanctions relief. Six world powers, including Russia, and Iran are working on a fuller deal that would place long-term restrictions on Iran's nuclear program in exchange for an end to all economic sanctions.

The six-month interim agreement, which went into effect in January and expires in July, allows Iran to continue exporting a total of 1 million barrels a day of oil to six countries: China, India, Japan, South Korea, Taiwan and Turkey. The promise didn't apply to Russia, which wasn't an existing customer of Iran's petroleum industry.

If Russia reaches the oil-for-goods contract with Iran, it would challenge Western efforts to secure a comprehensive agreement. U.S. Secretary of State John Kerry said Tuesday that Washington could impose sanctions if Russia and Iran move forward with the oil contract.

Ryabkov said he was unaware of any specific agreements, adding that a "normal exchange of opinions with Iranian colleagues has been going on to determine which sectors of economy are best suited for further development of ties."

He insisted that Russia wants to develop its ties with Iran and rejected the U.S. threat to impose sanctions.

"We don't think that any unilateral U.S. sanctions, no matter whom they target, are legitimate, and we reject such a stance," he said.