LONDON — The European Central Bank will continue to provide as much short-term liquidity as eurozone banks demand through at least the first quarter of next year, but ECB President Jean-Claude Trichet offered no hint Thursday on the size of additional purchases of troubled European sovereign bonds.

Trichet said the central bank’s bond-purchase program is “ongoing,” but offered no details on the scope of additional purchases.

Speculation had mounted over the possibility the central bank would unveil plans to make substantial purchases in an effort to arrest rising borrowing costs for peripheral eurozone nations.

The ECB “offered the bare minimum of support to the euro-area periphery” by extending its liquidity measures, which give commercial banks access to the ECB’s main refinancing and special one- and three-month-refinancing operations, said Chris Scicluna, an economist at Daiwa Capital Markets.

“Given the continued reliance of banks in the euro-area periphery on funding from the ECB, a decision not to do so simply would have generated even more financial instability than is already the case,” he said.

But Portuguese and other peripheral government bonds continued a rally in the wake of the ECB meeting, with strategists reporting that the central bank was a significant buyer.

The euro temporarily dipped after Trichet offered no details of future bond purchases, but then edged back into positive territory as spreads between peripheral and core bond yields narrowed.

The yield premium demanded by investors to hold Portuguese 10-year bonds over German bunds narrowed by around half of a percentage points to 3.36 percentage points, strategists said.

The ECB was “pretty active” in bond markets, said David Schnautz, fixed-income strategist at Commerzbank.

The purchases appeared to be a bid to further smooth volatility in peripheral markets, said Nick Stamenkovic, fixed-income economist at RIA Capital in Edinburgh, although Trichet’s remarks were largely a disappointment to the market.

The lack of commitment from Trichet shows that the ECB is leaving resolution of the crisis largely up to European political leaders, Schnautz said.

Market speculation this week had focused on the prospect that Trichet would explicitly signal that the central bank was prepared to ramp up purchases of distressed sovereign debt in an effort to halt sharp rises in peripheral bond yields. The speculation triggered a short-covering rally in peripheral bond markets on Wednesday.

Many economists had been skeptical of the scope for a dramatic announcement, noting that the Securities Market Program, put in place in May, has spawned divisions within the ECB’s 22-member Governing Council.

The ECB has bought 67 billion euros’ worth ($88.5 billion) of bonds since the program was launched.

Trichet emphasized that the program doesn’t amount to “quantitative easing,” the money-creating strategy employed by the U.S. Federal Reserve and the Bank of England, because the liquidity provided through the bond purchases is drained through other market operations.

“In other words, the ECB will continue doing what it has been doing to stabilize the banking system and calm the markets,” said Carl Weinberg, chief economist at High Frequency Economics in Valhalla, N.Y. “One can only expect that the market will not find hoped-for reassurances or policy initiatives in today’s comments.”

As expected, the ECB left its key lending rate unchanged at a record low 1 percent.

The ECB’s special liquidity measures, meanwhile, have become a crucial line of support for troubled banks in the euro zone that have been shut out of interbank funding markets.

Although ECB officials previously signaled they would consider further unwinding of its special liquidity measures at the December meeting, taking such action now would only have exacerbated the debt crisis, analysts said.