TOKYO, Japan -- Japan has sent a letter to the United States expressing concern about protectionism stemming from a "Buy American" plan in Washington's mammoth stimulus package, the top government spokesman said Thursday.

The letter, sent Wednesday to top aides of US President Barack Obama and the Senate's majority and minority leaders, echoes concerns by other US trading partners, including Canada and the European Union.

"The common responsibility shared by the two countries, which are the world's number one and number two economies, is to resist protectionism together," Chief Cabinet Secretary Takeo Kawamura told a news conference.

The letter was sent to US Secretary of State Hillary Clinton and head of the National Economic Council Lawrence Summers, as well as the Senate majority leader Harry Reid and minority leader Mitch McConnell, from Ichiro Fujisaki, the Japanese ambassador to Washington, Kawamura said.

The deepening global financial crisis and the failure to complete the World Trade Organization's long-running Doha round to free up global commerce, have raised fears that countries will block imports to protect jobs.

On Wednesday, the US Senate voted to soften the "Buy American" provision in its $900-stimulus bill, which originally allowed only US-made iron, steel and manufactured goods to be used in public works projects funded by the bill.

The vote for the amendment came after Obama expressed concern about the possibility of a trade war.

Asian stocks moved into positive territory at midday on the news, after opening lower on Thursday.

Japan is the fourth biggest exporter to the United States, after China, Canada and Mexico.