US Secretary of State Hillary Clinton and Pentagon chief Leon Panetta urged the Senate on Wednesday to ratify a UN treaty on the law of the sea, arguing it was vital for the country's economic and military interests.

President Barack Obama's administration has launched a fresh push for approval of the treaty, saying major US industries are losing out on commercial opportunities and Washington's diplomatic leadership is being undermined by the Senate's failure to ratify the convention.

"We are on the sidelines," Clinton told the Senate Foreign Relations Committee.

"We believe it is imperative to act now," she said.

Both Clinton and Panetta said the treaty was needed to bolster US credibility as it seeks to counter Beijing's claims in the South China Sea as well as Iran's threats over oil shipping in the Strait of Hormuz.

"You have all followed the claims that countries are making in the South China Sea. Although we do not have territory there, we have vital interests, particularly in freedom of navigation through that region," Clinton said.

"I can report from the diplomatic trenches that as a party to the convention, we would have greater credibility in invoking the convention's rules and a greater ability to enforce them," she said.

Panetta agreed, saying the United States, when confronting regimes in Iran or North Korea, often cites international law and demands these countries abide by global rules, but Washington's failure to ratify the treaty potentially damaged US credibility.

"For too long, the US has failed to act on this treaty. For too long, we have undermined our moral and diplomatic authority" by staying outside the treaty, he said.

The UN convention on maritime rights entered into force in 1994 but the US Senate has never ratified the treaty, despite support from successive presidents from both parties.

Some conservative Republicans oppose the agreement, arguing it could undermine US legal authority over oil and gas resources in the continental shelf.

Clinton said arguments against the treaty were based on "ideology and myth" and that it was telling that oil and mining companies as well as environmental groups had all endorsed it.

The convention would allow the United States to claim sovereignty over a continental shelf more than 200 nautical miles from shore, an area probably "more than one and one-half times the size of Texas," she said.

The treaty, ratified by 162 countries including China and Russia, offers "the firmest legal foundation upon which to base our global presence, on, above, and below the seas," Panetta said.

The US military's top officer, General Martin Dempsey, said the treaty would not hamper operations or intelligence gathering but "would provide us another way to stave off conflict with less risk of escalation."

Senator John Kerry, the chairman of the Senate Foreign Relations Committee, said he planned to put off a vote on the treaty until after presidential and congressional elections in November, to ensure a less politically-charged climate for debate.