"Intense and tough" negotiations between Iran and six Western nations Monday reportedly showed no signs of progress as the Islamic Republic refuses to yield in its uranium enrichment and demands economic sanctions against it be dropped.

The intransigence comes amid a New York Times report Thursday that espionage efforts to slow the Iranian march toward a nuclear weapon just haven't worked. Cyberattacks and suspected assassinations of nuclear scientists haven't stopped Iran from increasing its enrichment pace "and are now raising their production of a concentrated form close to bomb grade," the newspaper reported.

Monday's talks followed fruitless negotiations from April and come two weeks before a planned European Union oil embargo and enhanced American economic sanctions.

Israel, meanwhile, continues to warn that it will resort to military action than risk Iran developing nuclear weapons. The United States has vowed to keep a military option on the table, too, but has stuck by its sanctions and negotiation efforts.

A bipartisan group of 44 U.S. Senators on Friday wrote to President Obama saying it was time to cut off talks if Tehran refused to make concessions "and instead focus on significantly increasing the pressure on the Iranian government through sanctions and making clear that a credible military option exists."

Republican presidential nominee Mitt Romney said Sunday that he also would resort to striking Iran "if necessary to keep them from becoming a nuclear threat to the world" and that could be done without congressional approval.