WASHINGTON, August 5, 2008 (AFP) - The six world powers involved with talks on Iran's nuclear program will hold a conference call Wednesday to decide the next step, after Tehran defied calls to give a final answer to an incentives package, a US State Department spokesman said Tuesday.

'If we are not going to receive a clear response, a clear message from them, we are going to have no choice but to pursue additional measures,' spokesman Gonzalo Gallegos said.

Negotiators from United States, Britain, France, Germany, Russia and China 'have scheduled a conference call for tomorrow,' the spokesman said.

'So first we will see what happens with the rest of the day that is left in Europe, and then they will have their call tomorrow. We will see where we go from there.'