Iran nuclear talks are entering a critical stage. Here's what to look out for

Iran and the United States return to Vienna Tuesday for another attempt at reviving the 2015 nuclear agreement abandoned by the Trump administration. Here are some issues to watch out for that may give an indication of how the talks will progress.

There have been enough indications of progress from both sides to suggest that negotiations have entered a critical phase, but a number of issues remain unresolved.

Those include the scope of sanctions relief for Iran, sequencing of events, verification mechanisms, Iran's compliance process, Tehran's demand for guarantees the US will stick to the deal, says Sanam Vakil, senior research fellow at London's Chatham House think tank.