China and Iran, both subject to U.S. sanctions, signed a 25-year cooperation agreement in Tehran on Saturday.

Why it matters: China agreed to invest $400 billion in Iran's economy over that time period in exchange for a steady and heavily discounted supply of oil from the country, according to a draft of the agreement obtained by the New York Times.

What they're saying: “Relations between the two countries have now reached the level of strategic partnership and China seeks to comprehensively improve relations with Iran,” Chinese Foreign Minister Wang Yi said, according to Reuters.

• “Our relations with Iran will not be affected by the current situation, but will be permanent and strategic."

Thought bubble, via Axios' Bethany Allen-Ebrahimian: Iran has become ever more dependent on China, which has offered it a rare economic lifeline amid heavy U.S. sanctions.

The big picture: The agreement may deepen China’s influence in the Middle East and undermine U.S. efforts to keep Iran isolated due the unresolved dispute over Tehran's nuclear program, per the NYT.

• Iran's leaders indicated in January that they're willing to strike a nuclear agreement with the Biden administration.
• But Biden said last month that Iran will first have to stop enriching uranium above levels set by the 2015 Iran nuclear agreement before the countries can revive the deal.
• The U.S. reentering the 2015 Iran nuclear deal remains unpopular with U.S. ally Israel.

Go deeper ... 43 senators to Biden: Use "full force" of diplomatic, economic tools to check Iran