FRANKFURT (Reuters) - The European Central Bank increase bond purchases by nearly half last week, ramping up its stimulus efforts to keep a lid on borrowing costs and convince sceptical investors it would do what it took to restrain bond yields. The ECB was finally putting its money where its mouth is after announcing on March 11 that it would step up its Pandemic Emergency Purchase Programme over the coming quarter to ensure credit stayed cheap in the virus-stricken euro zone. The euro zone’s central bank bought 28 billion euros worth of bonds last week across its stimulus programmes, a 48% increase over the previous week and the biggest weekly amount since Dec. 4. PEPP accounted for the bulk of the purchases at 21.05 billion euros, a 50% increase on the week. Investors have been doubting the ECB’s resolve to tackle a recent increase in bond yields in the euro zone, which largely reflected higher growth and inflation expectations in the United States rather than a brighter outlook for the bloc. Dutch central bank governor Klaas Knot, a long-standing critic of the benefit of bond purchases, said on Monday the larger purchases were temporary and only meant to reduce borrowing costs until growth and inflation pick up. But ECB President Christine Lagarde warned the euro zone’s economic outlook was marred by uncertainty about the evolution of the COVID-19 pandemic and the speed of the vaccination campaign.