The warning came as an artillery shell slammed into a village in central Syria and killed 11 people, including a woman and six of her children, activists said.
Walid al-Moallem said sending more weapons to the opposition would also hinder efforts to convene a peace conference in Geneva to work on a negotiated solution. He said his country remains ready to take part but added President Bashar Assad will not step down. His resignation is a key opposition demand to be raised in any talks with Damascus.
US Secretary of States John Kerry, who took part in the conference, would not disclose details of the aid, saying only that it would re-balance the fight between the rebels and the government. Assad's better-equipped forces are increasingly backed by fighters from Lebanon's Hezbollah militant group.
"If they dream or are delusional about achieving a balance with the Syrian Arab Army, I think they need to wait years, and this won't be achieved," al-Moallem said.
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The Friends of Syria gathering in Doha was Kerry's first meeting with his counterparts on the practicalities of assisting the Syrian rebels since President Barack Obama announced that the US would send arms to the opposition. The decision came despite concerns that the weapons could fall into the hands of Islamic extremists in the rebel coalition.
Obama's change of policy was partly based on a US intelligence assessment that Assad had used chemical weapons, but Kerry expressed deeper concern about Assad's foreign support. He said that Iranian as well as Hezbollah fighters had joined the war.
The foreign minister said his regime was willing to take part in a peace conference, but would go to Geneva not to hand over power to the other side but rather to establish "a real partnership" and a national unity government that includes representatives of all Syrian society.