BEIJING (Reuters) - China accused the United States of "unilateral bullying" in calling on allies to impose tariffs on China over its purchase of Russian oil, fueling tensions as Chinese and U.S officials meet in Spain to try to resolve trade disputes.

China opposes Washington's request that the Group of Seven and NATO countries impose secondary tariffs on Chinese imports over its purchase of Russian oil, China's commerce ministry said on Monday, calling it "a classic example of unilateral bullying and economic coercion".

Officials from China and the U.S. on Monday entered a second day of talks in Madrid to try to seek common ground on issues including tariffs and a U.S. demand for divestment from TikTok by Chinese owner Bytedance.

Adding to tensions between the two countries, China's market regulator on Monday said a preliminary investigation found U.S. chipmaker Nvidia had violated the country's anti-monopoly law.

Trade ties have soured between the world's two biggest economies despite a fragile tariff truce reached in May and extended in August which prevented tariff rates on each other's goods from reaching three-digit levels.

But negotiators from the two sides still face thorny topics such as U.S. curbs of tech and chip exports, China's support for Russia as well as what Washington sees as insufficient efforts to stem the flow of precursor chemicals of fentanyl into the U.S.

In its statement, the Chinese ministry urged the U.S. to be "prudent in words and deeds" and to solve differences through equal dialogue.