The Obama administration stepped up a year-long pressure campaign to halt Iran’s nuclear ambitions by imposing sanctions Tuesday on seven foreign companies it says are illegally helping Iran acquire gasoline and other refined petroleum products.

The seven firms targeted for alleged business ties to Iran include an Israeli company accused of helping provide an oil tanker to Iran last fall. The company, Ofer Brothers Group, denied the allegation, saying in a prepared statement, “We have never sold ships to Iran.”

The State Department announced the new penalties as part an ongoing effort to squeeze Iran financially to force it to give up its nuclear ambitions. An expanded sanctions regime adopted by the White House last year authorizes action against U.S. and foreign firms that do business with Iranian energy companies.

“We need to continue to keep the pressure on Iran,” a senior administration official said in announcing the sanctions, speaking on the condition of anonymity to discuss sensitive matters. “It has clearly had an impact on the their economy.

In addition to Ofer Brothers, the companies targeted are the Venezuelan firm Petroleos de Venezuela; an Iranian/Jersey Island-owned firm called Petrochemical Commercial Co. International; and shipping or tanker companies owned or co-owned by businessmen in Singapore, the United Arab Emirates, Iran and Monaco. All are subject to penalties that include the forfeiture of U.S. government contracts and the loss of U.S. import and export licenses.

The new measures come days after the European Union slapped sanctions on banks and shipping companies accused of illegally doing business with Iran. Western powers have been ratcheting up the pressure on Iran to forces its leaders to resume negotiations on curtailing its nuclear program. The United States accuses Iran of secretly pursuing atomic weapons, while Iran insists that its nuclear program is solely for energy production.

Separately, a report Tuesday by the U.N. nuclear watchdog found that Iran made significant gains in recent months in the production of enriched uranium. In its routine monitoring of Iran’s nuclear facilities, the International Atomic Energy Agency noted a 30 percent jump in Iran’s output of low-enriched uranium compared with last summer, when Iran was struggling to contain the damage from a cyberattack on its main nuclear complex.

Iran had already amassed enough enriched uranium to make — if it chooses to — at least two nuclear bombs, and since February it has added 1,100 pounds of low-enriched uranium to its stockpile, said David Albright, a former U.N. nuclear inspector who analyzed the IAEA report.

“This is the highest level that Iran has ever achieved,” Albright said.

The watchdog group also criticized Iran anew for failing to adequately explain past nuclear experiments by Iranian scientists that appear related to nuclear warhead design. The IAEA, citing documents provided by several intelligence agencies, has demanded that Iran come clean about research projects that focused on, among other things, how to make high-precision detonators of the kind used to set off a nuclear chain reaction.

The IAEA report listed several new details about the alleged experiments. In one exercise, the scientists were exploring how to use a detonator to implode a spherical object. Similar technology is used to compress a spherical plutonium “puck” in traditional nuclear warhead design.

IAEA officials have sought explanations about the research projects since they first came to light four years ago. In a letter to senior Iranian nuclear officials this month, IAEA Director General Yukiya Amano issued a new appeal for answers, saying the U.N. agency is “unable to provide credible assurances” about the peaceful nature of Iran’s nuclear program, the report said.