Indian government bonds tumbled on Wednesday, reversing most of the previous session's sharp rally, as a fresh jump in oil prices and US Treasury yields rattled investors ahead of the Federal Reserve's policy decision.The benchmark 6.94% 2036 bond yielded 7.4772% at 10:50 a.m. IST, up 4 basis points from its previous close. The 10-year yield had dropped by nearly five basis points on Tuesday, its steepest single-day decline in three months, after failing to breach 7.50%.On the day, Brent crude climbed 1.5% to $115.40 a barrel after an attack on a Saudi Arabian biggest refinery revived concerns over supply disruptions from the conflict involving Iran and its proxies.India, the world's third-largest oil importer, is particularly vulnerable to oil-price shocks.