UNITED NATIONS, Feb 7 (Reuters) - The United Nations Security Council on Sunday strongly condemned North Korea's latest rocket launch and promised to take punitive steps, while Washington vowed to ensure the 15-nation body imposed "serious consequences" on Pyongyang as soon as possible.

"The members of the Security Council strongly condemned this launch," Venezuelan Ambassador Rafael Dario Ramirez Carreno, president of the council this month, told reporters. He said the launch was "a serious violation."

He added that the 15-nation council "restated their intent to develop significant measures in a new Security Council resolution in response to the nuclear test" in January, as well as Sunday's rocket launch. He said they would work "expeditiously."

Standing alongside her Japanese and South Korean counterparts, U.S. Ambassador Samantha Power told reporters: "We will ensure that the Security Council imposes serious consequences. DPRK's (North Korea) latest transgressions require our response to be even firmer."

North Korea has been under U.N. sanctions since its first nuclear test in 2006. It has conducted three more atomic tests since then, including the one last month, along with numerous ballistic missile launches.

The sanctions ban its work in nuclear weapons and ballistic missiles, blacklist a number of individuals and entities and bar the country's leadership from importing luxury goods.

The United States and China began discussing a resolution to expand the existing sanctions after Pyongyang's atom bomb test on Jan 6. Power said she hoped the council would have a draft resolution to vote on "as quickly as possible."

"It is urgent and overdue," she said.

"We are hopeful that China, like all council members, will see the grave threat to regional, international peace and security, see the importance of adopting tough, unprecedented measures, breaking new ground," Power added about her hopes for China in discussions on new sanctions on Pyongyang.

Diplomats say Washington is closely consulting with Japan, South Korea, Britain and France on its discussions with China, while Beijing is keeping in close contact with fellow veto power Russia.

Japanese Ambassador Motohide Yoshikawa said the draft under discussion would have "much more strengthened measures" against Pyongyang.

Speaking on condition of anonymity, one senior Western diplomat said he hoped the council would be able to vote on a new sanctions resolution this month.

He said the Americans had been pushing for tough new measures that went beyond targeting North Korea's atomic weapons and missile programs, while China wanted any future steps to focus on the question of nonproliferation.

One diplomat told Reuters that Washington was hoping to tighten international restrictions on North Korea's banking system, while Beijing was reluctant to support that for fear of worsening conditions in its impoverished neighbor.

"There will eventually be a sanctions resolution," the diplomat said. "China wants any steps to be measured but it wants the council to send a clear message to DPRK (North Korea) that it must comply with council resolutions."

China expressed regret and concern over Sunday's rocket launch, which employed ballistic missile technology. China is North Korea's main ally but it disapproves of its nuclear weapons program.

Speaking to reporters ahead of the closed-door session, France's U.N. ambassador, Francois Delattre, described North Korea's launch as an "outrageous provocation."

British Foreign Secretary Philip Hammond said on Sunday he had spoken with his Japanese counterpart, Fumio Kishida, and both had agreed the council should take strong action.

(Editing by Jonathan Oatis)