The US has added seven nations to its latest roster of countries exempted from its sanctions on oil trade with Iran on June 11, but China and Singapore did not make it on the list.

Countries granted the waiver include India, Malaysia, South Korea, South Africa, Sri Lanka, Turkey and Taiwan for significantly reducing the amount of oil they bought from Iran, said US Secretary of State Hilary Clinton at a high-level meeting in Washington in June.

The US is poised to impose the punitive measure on June 28, when banks of nations that settle oil trades with Iran will no longer be able to access the US financial system.

The US-led sanction, which was signed into law last December, aim at cutting off Iran’s key foreign income and crushing the Persian Gulf state’s nuclear aspirations.

The Obama administration has yet to clearly define what constitutes “significant” reduction of oil purchase from Iran that can lead to the six-month renewable exemption, but sources aware of the decision told Bloomberg that it includes a host of considerations such as availability of alternative energy sources and each country’s fuel demand.

Japan and ten European Union nations were granted the waiver on March 20.

Whether China, Iran’s number one buyer, would be granted the waiver is a testy political question as Obama does not want the issue to hamper his chances to win the next presidential election on November 6 by being blamed for triggering an energy crisis that sinks the world further into the recession.

China has not made clear in public whether it will concede to the US move. And Reuters reported that China received clandestine oil imports from Iran with cargoes that disabled tracking devices.

With all that said, experts believe it is yet to be seen how the sanction will impact global oil supply and crude price.

Many nations have prepared for the impending sanction and scaled back their imports of Iranian oil.Major oil producers such as Saudi Arabia, Iraq and Libya have pumped out more oil to fill the vacuum left by Tehran.

As Spain made a plea to bail out its banks and the eurozone crisis impedes economic growth and curbs energy use, prices for crude for July delivery fell $1.40 to $82.70 a barrel on the New York Mercantile Exchange, an eight-month low.

Another European Union-led sanction against Iran is set to prohibit access to tanker insurance which may halt shipments from July 1. It’s not popular to be friendly with Iran right now.