* Votes come day after Obama announced fresh Iran sanctions
* Measure still needs Obama signature (Updates with Senate passage)
By Roberta Rampton
WASHINGTON, Aug 1 The U.S. Congress overwhelmingly passed a new package of sanctions against Iran on Wednesday that aims to punish banks, insurance companies and shippers that help Tehran sell its oil.
The legislation, agreed to by senior lawmakers of both parties, "seeks to tighten the chokehold on the regime beyond anything that has been done before," said Republican Representative Ileana Ros-Lehtinen, chair of the House Foreign Affairs Committee.
The bill now heads to the White House for President Barack Obama's signature. It builds on oil trade sanctions signed into law by Obama in December that have prompted Japan, South Korea, India and others to slash their purchases of Iranian oil.
The United States, European Union, and other Western nations are trying to stop Iran's suspected pursuit of nuclear weapons. Iran says its nuclear program is for peaceful purposes.
"We are taking another significant step to block the remaining avenues for the Iranians to fund their illicit behavior and evade sanctions," said Democratic Senator Tim Johnson, chairman of the Senate Banking Committee.
The Senate passed the sanctions bill unanimously and the House passed it on a vote of 421-6.
Lawmakers from both parties said they are prepared to take additional steps.
"There is more we can do, more that we will do if Iran doesn't end its nuclear weapons program verifiably and completely," said Representative Howard Berman, the top Democrat on the foreign affairs panel.
The bill was endorsed by the American Israel Public Affairs Committee, a powerful pro-Israel lobby group, which said the measure when coupled with existing U.S. sanctions "represents the strongest set of sanctions to isolate any country in the world during peacetime."
Obama announced U.S. sanctions on Tuesday against foreign banks that help Iran sell its oil, specifically citing China's Bank of Kunlun and an Iraqi bank.
The sanctions followed criticism from Republican presidential challenger Mitt Romney that the White House had failed to act strongly enough.
China's Foreign Ministry said the sanctions announced by Obama would hurt cooperation between China and the United States.
"The U.S. has invoked domestic law to impose sanctions on a Chinese financial institution, and this is a serious violation of international rules that harms Chinese interests," ministry spokesman Qin Gang said in a statement.
The United States gave China, Iran's top customer for oil, a six-month reprieve from sanctions in June, saying it had cut its purchases. That decision sparked criticism in Congress. China's imports had fallen early in the year due to a pricing dispute, but have since rebounded. (Additional reporting by Samson Reiny and Donna Smith; Editing by Stacey Joyce and Jackie Frank)