Greece’s lenders scrutinized a proposal seen as a potential breakthrough on a last-minute bailout deal on Tuesday, but the concerns of some creditors suggest that days of tough negotiations lie ahead before an agreement can be clinched.

After months of deadlock, European leaders emerged the day before from a string of meetings in Brussels optimistic over a Greek concession on pensions. Officials hope that eurozone finance ministers can sign off on a policy package agreed to between Greece and the institutions overseeing its...