Iran has acknowledged that Western sanctions over its controversial nuclear program have reached their toughest level, as the European Union begins a total ban on Iranian oil imports.

In remarks published Sunday by Iranian state media, Vice President Mohammad Reza Rahimi said Western powers unfairly have imposed what he called the “most severe” sanctions on Iran to date. He appealed to the Iranian people to help the government fight the impact of those measures.

The EU ban on Iranian oil took full effect on Sunday, after being implemented gradually since the start of this year. The 27-nation bloc also began enforcing a ban on European insurance for Iranian oil shipments to other nations.

The Obama administration welcomed the European moves, which came several days after it tightened U.S. sanctions against foreign companies doing business with Iran. White House Press Secretary Jay Carney said the EU sanctions are a “substantial additional commitment” to a diplomatic strategy of forcing Iran to choose between isolation or meeting its international obligations.

Iran has been enriching uranium for what it calls a civilian energy and medical research program, in defiance of several U.N. Security Council resolutions. Western powers that backed those resolutions want Iran to stop enrichment activities that they fear could be used to make nuclear weapons.

Carney said Iran still “has an opportunity” to pursue substantive negotiations to resolve the issue. He said that opportunity begins with expert-level talks in Istanbul on Tuesday. Iran and the five permanent members of the U.N. Security Council plus Germany failed to reach any breakthroughs in three rounds of high-level negotiations held earlier this year.

Other Iranian officials tried to reassure the public that they are working to ease the impact of the latest Western sanctions. Iranian central bank governor Mahmud Bahmani said the government has plans to counter the sanctions and has enough foreign currency to meet Iran’s import needs.

Oil Minister Rostam Ghasemi said he does not see the sanctions as a problem because Iran has faced similar measures for years and “nothing happened,” as he put it. He said Iran already has found new buyers to replace EU nations who imported Iranian oil. The EU previously bought about 18 percent of Iran’s oil exports, which are the main source of that nation’s revenue.

In another sign of Iranian defiance toward the West, an Iranian Revolutionary Guards commander said his forces will begin three days of missile tests on Monday, with missiles being fired at desert targets made to resemble enemy air bases in the region. Brigadier General Amir Ali Hajizadeh warned that Iranian missiles would “obliterate” Israel if that country attacks Iran.

Israel sees a nuclear armed Iran as a threat to its existence and says it could take military action to prevent Iran from developing a nuclear weapon capability. The United States refuses to rule out military action to stop Iran from obtaining a nuclear bomb.