The United States is asking one of its key Asian allies, South Korea, to join it in toughening sanctions against Iran. This comes in response to rising concern about Iran's nuclear program.
As the United States and countries of the European Union discuss restricting Iran's lucrative oil exports, other nations are being approached to curtail business relations with Tehran.
The International Atomic Energy Agency of the United Nations last month issued a report outlining more evidence Iran is working to design a nuclear weapon.
Tehran says its nuclear activities are only to generate electricity.
But, since the report, Western nations have taken a series of actions aimed at further isolating Iran. As part of those diplomatic maneuvers, the U.S. State Department's adviser for non-proliferation and arms control, Robert Einhorn, is in the Republic of Korea this week.
Einhorn says he understands South Korea, with scant fossil fuel resources, may be reluctant to significantly reduce purchases of Iranian crude oil. But he says South Korea and other allies in a similar situation can switch suppliers of petrochemicals.
"I think the ROK [South Korean] government recognizes the importance at this particular juncture of sending a clear, unified message to Iran," said Einhorn. "We have, I think, gotten a positive reaction the ROK government is continuing to give consideration to what additional measures it wishes to take."
About 10 percent of South Korea's total oil imports come from Iran.
That accounts for half of the total trade between the two countries. But South Korea is also taking into consideration the close relationship on advanced weapons technology -- possibly including nuclear -- between Iran and Seoul's rival neighbor, North Korea.
The United States maintains more than 28,000 military personnel in South Korea. American troops have been posted here since the Korean War of the early 1950's. The two Koreas technically remain at war as no peace treaty was signed.
Another item on Einhorn's agenda this week is discussions on revising a 1974 agreement between Seoul and Washington on nuclear energy. The accord, which expires in three years, currently prohibits South Korea from reprocessing spent nuclear fuel that could yield plutonium to make nuclear weapons.
Einhorn is among several U.S. government officials visiting Seoul in the next few days for foreign policy discussions.
The new special representative for North Korean policy, Glyn Davies, is also to hold talks. He is expected to be joined by the top U.S. nuclear negotiator on North Korea, Clifford Hart. Hart's predecessor, Sung Kim, recently became the U.S. Ambassador to South Korea.
Also due to arrive in Seoul is the deputy assistant defense secretary for East Asia, Derek Mitchell. He is also a special envoy for Burma.
In wake of Secretary of State Hillary Clinton's historic visit to Burma last week, Mitchell is expected to ask for South Korea's help to pressure Burmese officials to cut any remaining questionable relationships with North Korea.
U.S. officials say, among the conditions for America's relationship with Burma to improve is the Southeast Asian country coming clean about cooperation with Pyongyang on missile and nuclear development.