SEOUL - A senior US official urged energy-starved South Korea to cut imports of Iranian petrochemicals and reduce crude oil imports from the Gulf state, as Washington applies more pressure on Tehran.

Iran's nuclear ambitions, its claim to have shot down a US spy drone in its airspace on Sunday and last week's storming of the British embassy in Tehran by protesters have contributed to a sharp increase in tension in the region.

Robert Einhorn, the US State Department senior adviser for non-proliferation and arms control, warned that Iran's nuclear programme was becoming increasingly worrying and an urgent diplomatic solution needed to be found.

Einhorn told a news conference in Seoul the United States had been in contact with South Korea to discourage imports of petrochemicals, but stopped short of suggesting specific sanctions.

South Korean sources said last month that Seoul might ban petrochemical imports from Tehran, although admitted the measure would have only limited impact.  

South Korea imported $350 million in Iranian petrochemicals last year, while exporting $450 million of its petrochemicals to Iran. That would represent a small part of South Korea's global trade in petrochemicals, which last year totalled $49 billion.  

Einhorn said Washington had not asked Seoul, the world's fifth-largest crude importer, to stop purchases of crude oil from Iran. South Korea imports about 10 percent of its crude from Tehran.

"But we discourage countries from continuing to import crude oil in large quantities," added Einhorn, acknowledging that at the present time "pressure was tight" on the oil market.

"We are conscious of energy security needs of countries like the Republic of Korea and don't want to interfere with those needs," he said of Asia's fourth largest economy.

Einhorn said he had received a positive response during talks with South Korean officials about tightening sanctions, adding Seoul was considering what additional measures to take.

Western nations last week significantly tightened sanctions against Iran, with the European Union expanding an Iranian blacklist and the US Senate passing a measure that could severely disrupt Iran's oil income.  

"Iran is violating international obligations and norms. It is becoming a pariah state," Einhorn said.

"The situation in Iran has become more and more worrisome. The timeline for its nuclear programme is beginning to get shorter, so it is important we take these strong steps on an urgent basis.    

"If we do not, pressures will grow for much stronger actions. The US favours a diplomatic solution pressure, but if we cannot achieve a diplomatic solution soon, inevitably interests will grow in a different kind of solution. That is why we need to act soon."  

The United States and Israel have not ruled out military action against Iran's nuclear facilities if diplomacy fails to resolve a dispute over a programme they suspect is aimed at developing atomic weapons.  

Iran denies it is seeking nuclear weapons and says its nuclear programme is for energy purposes only. It says it would respond to any strike by attacking Israel and US interests in the Gulf.