It remains unclear whether a nuclear deal is possible, and talks may have to continue after Tuesday's midnight deadline to agree a framework accord, a German diplomatic source said.
"The whole day (has seen) difficult negotiations on the decisive issues. The atmosphere is changeable, (there are) tough negotiations on do-able solutions that are acceptable for both sides. It remains unclear whether we will succeed... It is still too early to talk about stopping the clock, but this may in fact be necessary," the source said.