(Adds details of agreement, names of companies)
MEXICO CITY, May 21 (Reuters) - A $400 billion gas supply deal signed between Russia and China on Wednesday was the result of 10 years of negotiations and is not tied to diplomatic tensions in Ukraine, U.S. Secretary of State John Kerry said.
"We don't see any relationship whatsoever to an agreement with respect to gas and energy supplies between Russia and China that they've been working on for 10 years, for 10 years," Kerry told a news conference during a visit to Mexico City.
"This isn't new. This isn't a sudden response to what's been going on," he added.
The deal between state-controlled entities Gazprom and China National Petroleum Corp will link Russia's huge gas fields to Asia's booming market for the first time - via thousands of miles of pipeline across Siberia.
The agreement was a political triumph for Russian President Vladimir Putin, who is courting partners in Asia as those in Europe and the United States seek to isolate him over Moscow's annexation of Ukraine's Crimea peninsula.
Kerry also noted that Washington was paying close attention to whether Russia would fulfill its pledge to bring troops near the Ukrainian border back to their bases.
"If that happens, and we're watching carefully, that's extremely constructive," he said. "It's positive." (Reporting by Patricia Zengerle; Editing by Lisa Shumaker)