Washington, Jul 2 (DPA): US President Barack Obama signed a law Thursday imposing tough new sanctions on Iran and making it harder for the Islamic state to fund its nuclear programme.

The sanctions were drawn up and overwhelmingly approved last week by Congress. They expand existing sanctions on Iran's energy and banking sectors and could penalise companies who do business with the Iranian government.

"We are ratcheting up the pressure on the Iranian government for its failure to meet its obligations," Obama said.

Iran has been under mounting pressure for its refusal to comply with UN Security Council resolutions demanding Tehran suspend uranium enrichment, a process that could be used to make nuclear weapons but that Iran maintains is solely for producing energy.

The US and Western allies suspect Iran is trying to acquire the ability to develop nuclear weapons.

"With these sanctions - along with others - we are striking at the heart of the Iranian government's ability to fund and develop its nuclear programmes," Obama said.

The Security Council recently passed a fourth round of sanctions on Iran. The US Treasury Department followed with more sanctions earlier this month and the European Union is designing its own package of sanctions. The congressional legislation is the latest effort.

It would allow the United States to penalise companies who contribute to Iran's energy sector. It would also require any company contracted by the US government to certify it is not doing business with the Iranian government or risk losing contracts. The legislation provides the president with the power to enact a waiver on the sanctions for a 12-month period.