'Disastrous consequences'
 
The letter was signed by Robert Gard, a former senior military advisor, Jack Shanahan, an ex-director of the Center for Defense Information and Joseph Hoar, a former commander-in-chief of US Central Command.
 
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"The US has recently increased the regularity and vehemence of its verbal attacks on Iran"
The three urge the US government to "engage immediately in direct talks with the government of Iran without preconditions.
 
"There is time available to talk, we must ensure that we use it," they said.
 
Attacking Iran "would have disastrous consequences for security in the region, coalition forces in Iraq and would further exacerbate regional and global tensions" they said.
 
The three men have joined previous petitions calling on the Bush administration to change course in its policy on Iran.
 
Regional instability
 
The US administration has recently increased the regularity and vehemence of its verbal attacks on Iran, prompting fears that it could be preparing for a military strike against the Islamic state.
 
Washington has also sent a second aircraft carrier to the Gulf, a move seen as a warning to Iran, which the US accuses of seeking nuclear weapons and fuelling instability in Iraq and elsewhere in the Middle East.
 
It has offered to hold direct talks with Iran but only once Tehran halts its push to produce nuclear fuel through uranium enrichment.
 
Iran denies the claims and says its nuclear programme is for its own energy use.
 
On Friday, Robert Gates, the US defence secretary, said Washington was not planning for war with Iran, but again accused Tehran of supplying bombs for attacks on US troops in Iraq.