FRANKFURT, Aug. 2 — The European Central Bank signaled Thursday that it would raise interest rates in September to curb inflation amid an expanding economy. Its president, Jean-Claude Trichet, also vowed to pay “great attention” to volatile developments in global financial markets.

Mr. Trichet, who does not usually speak to the news media in August, made an appearance after the bank voted Thursday to keep borrowing costs unchanged. He said the bank would exercise “strong vigilance” to ensure that higher inflation does not appear. The Bank of England also left its benchmark rate unchanged Thursday, at 5.75 percent.

Mr. Trichet’s use of that phrase suggested the bank’s benchmark interest rate would rise a quarter of a percentage point, to 4.25 percent, at the next policy meeting, in September — the ninth increase since the end of 2005.

Inflation is now running slightly lower than the bank’s target of below, but close to, 2 percent.

“That they are willing to preannounce a rate increase means they are very confident of the fundamentals,” said Erik Nielsen, chief European economist at Goldman Sachs in London. “They could have gotten out of it if they wanted to, but the market reaction showed they do not need to.”