The Syrian conflict has split the Middle East along sectarian lines. Shi'ite Muslim Iran has supported Assad and his Alawite minority against mainly Sunni rebels, some of them Islamists, who have backing from Gulf Arab states.
In Tehran, a foreign ministry spokesman said: "We want to strongly warn against any military attack in Syria. There will definitely be perilous consequences for the region.
"These complications and consequences will not be restricted to Syria. It will engulf the whole region."
Syrian foreign minister Moualem, who insisted the government was trying to help the U.N. inspection team, told a news conference in Damascus that Syria would hit back if attacked.
"We have means of defending ourselves, and we will surprise them with these if necessary," he said. "If we face aggression, we will defend ourselves. We will not hesitate to use any means available. But I will not specify what those would be."
Assad's forces made little or no response to three attacks by Israeli aircraft earlier this year which Israeli officials said disrupted arms flowing from Iran to Lebanon's Hezbollah.
China, which has joined Moscow in vetoing measures against Assad in the U.N. Security Council, is also sceptical of Western readiness to use force to interfere with what it sees as the internal affairs of other countries. Beijing's official news agency ran a commentary on Tuesday recalling the United States invaded Iraq in 2003 on the grounds that it possessed banned weapons - which were never found.
"The recent flurry of consultations between Washington and its allies indicates that they have put the arrow on the bowstring and would shoot even without a U.N. mandate," the Xinhua agency said. "That would be irresponsible and dangerous."
The continued presence of United Nations experts in Damascus may be a factor holding back international military action.
A U.N. statement said the investigators had put off a second visit to the affected areas until Wednesday to prepare better.
Some residents of the capital are getting anxious.
"I've always been a supporter of foreign intervention but now that it seems like a reality, I've been worrying that my family could be hurt or killed because they live near a military installation," said one woman, Zaina, who opposes Assad.