The United States said on Wednesday that it is continuing conversations with some countries over their reduced dependence on Iran's crude oil.

State Department spokeswoman Victoria Nuland said whether more countries can be exempted from U.S. new tough sanctions on Iran " depends on the progress of those countries."

"You know where we are, that we'd like to see all of the countries that trade heavily in Iranian crude reduce their numbers, " she told reporters at a regular news briefing, saying conversations are continuing with India, China, Turkey, South Korea and other countries.

"The concern is that all countries should do as much as possible to wean themselves of Iranian crude, that at a time when we're trying to convince Iran to definitively demonstrate that their (nuclear) program is not a weapons program, we have to maximize the pressure," she explained.

Washington announced on Tuesday that it would initially exempt Japan and 10 European nations from its sanctions on Iran, saying these countries have "significantly" reduced their oil imports from the Islamic republic.

President Barack Obama signed a bill at the end of last year, expanding U.S. sanctions against Iran to cover its central bank and financial sector, a move that allows penalties on foreign banks that settle oil imports with the Iranian central bank.

The law, however, offers waivers to firms from countries that significantly reduce their crude imports from Iran. Washington has sent officials to Japan, South Korea, India and other countries to persuade them to reduce their dependence on Iran's oil.

"The lifeline to the Iranian regime is its crude oil supplies. So this legislation has had the effect of crystallizing minds and causing countries to look hard at their Iranian relationship around the world," Nuland said.

She claimed that "very significant" progress has been made by countries in stifling Iran's oil exports, which in turn is having "a very important" impact on the country.

"This is actually leading us in the right direction in terms of trying to solve this diplomatically in the time and space that we have," she said.

She noted that Japan and the 10 European nations -- Belgium, Britain, the Czech Republic, France, Germany, Greece, Italy, the Netherlands, Poland and Spain -- will have the exemption until Sept. 16, when their renewable preferential treatment will be looked at again.