FRANKFURT, March 11 (Reuters) - The European Central Bank said on Thursday that it was going to boost its bond purchases over the next quarter, in a likely attempt to keep borrowing costs low for a euro zone economy still struggling with the COVID-19 pandemic. The ECB did not make changes to policy, having nearly a trillion euros of firepower left to buy bonds and keep credit cheap for debt-laden governments, households and firms across the 19 countries that share the euro currency. But it sought to dispel investor doubts about its resolve to stem any further rally in bond yields, which have risen in recent weeks due largely to external factors such as higher inflation expectations in the United States, rising oil prices and supply disruptions related to the pandemic. “Based on a joint assessment of financing conditions and the inflation outlook, the Governing Council expects purchases under the PEPP over the next quarter to be conducted at a significantly higher pace than during the first months of this year,” the ECB said. Although no policy change is expected for months and possibly all year, the ECB is under market pressure to clarify how far it is prepared to let bond yields rise before intervening and which other metrics of financing costs it is monitoring. Policymakers have been divided on the merit of increased bond purchases, with some saying the recent rise in yields was not justified by better economic prospects while others said it could even be welcome. Investors will be looking for answers from ECB President Christine Lagarde’s 1330 GMT news conference, at which she will also unveil new economic projections. These are expected to show slightly higher inflation this year but lower GDP growth as economic activity in the euro zone remains curtailed by pandemic-fighting restrictions and the vaccine rollout proceeds slowly. With Thursday’s decision, the ECB maintained its Pandemic Emergency Bond Purchase (PEPP) quota at 1.85 trillion euros and expects the buys to run at least until March 2022. The bank also left its deposit rate at minus 0.5% and kept the door open to further cuts if needed. It will also continue to provide banks with long term loans at a rate of minus 1%. (Reporting By Francesco Canepa; Editing by Catherine Evans)