OPEC and its allies agreed to make another modest output increase in March, sticking to their plan even as the failure of several members to deliver the scheduled monthly supply hikes stokes a rally in crude prices. After a brief meeting on Wednesday, the 23-nation coalition rubber-stamped the nominal revival of 400,000 barrels a day for March, according to a statement posted on its website. Widespread difficulties in restoring supplies increasingly place the burden on the group’s Gulf nations: Saudi Arabia, the United Arab Emirates, Iraq and Kuwait. That’s leaving traders anxious over the spare capacity available to cover any disruptions, whether deeper losses in Libya or another attack like last month’s drone strike in Abu Dhabi.