Israel's prime minister has said Iran is building sites in Syria and Lebanon to produce precision-guided missiles.
Benjamin Netanyahu accused Iran of turning Syria into a "base of military entrenchment as part of its declared goal to eradicate Israel".
Iran's forces are supporting President Bashar al-Assad in Syria's war and it backs Lebanon's Hezbollah movement.
Mr Netanyahu's comments came as he held talks with UN Secretary General António Guterres in Jerusalem.
Mr Guterres is on his first visit to the region since taking office in January.
Mr Netanyahu gave no details about the sites Iran was allegedly building to manufacture missiles, but he warned "this is something Israel cannot accept".
Two weeks ago, the Israeli satellite imagery company ImageSat International published photographs it said appeared to confirm a report by a Syrian pro-opposition newspaper that a missile factory was under construction in north-western Syria under Iranian oversight.
ImageSat International said the facility in Wadi Jahannam, near the Mediterranean coastal town of Baniyas, bore a close resemblance to a missile factory near Tehran.
There was no comment from Iran, which calls for the Jewish state's eradication.