Ban, who said he has spoken with Syrian President Bashar al-Assad several times, most recently four or five days ago, said some promises of change made by Assad had fallen short.
He said leaders in the troubled Arab world had a "once-in-a-generation opportunity" to meet their people's aspirations for basic freedoms, warning that gains in both Tunisia and Egypt remain fragile amid renewed unrest there.
U.N. aid workers and rights monitors must be allowed into the southern Syrian city of Deraa, cradle of the uprising against Assad, and other cities so as to assess the situation and needs of the civilian population, Ban said.
"I urge President Assad to heed the calls of the people for reform and freedom and desist from excessive force and mass arrests of peaceful demonstrators, and to cooperate with the human rights monitors," he told a news conference in Geneva.
Army tanks shelled a residential district in Homs on Wednesday, said a rights campaigner in Syria's third city that has emerged as the most populous centre of the seven-week-old public defiance of Assad's rule.
Assad initially responded to the unrest, the most serious challenge to his 11-year grip on power, with promises of reform. He granted citizenship to stateless Kurds and last month lifted a 48-year state of emergency. But a security clampdown went on.
"Listen to the people"
"On the political side again, I have been urging him (Assad) to listen more attentively, carefully and engage in inclusive dialogue with the people," Ban said.
"Again I urged him to take bold and decisive measures before it is too late and I will continue to do that."
Syrian human rights groups say that up to 800 people have been killed since the protests erupted in March, and hundreds more have been arrested.
Turning to Libya's civil war, Ban said he had spoken to Prime Minister Al-Baghdadi Ali al-Mahmoudi on Tuesday and called for an "immediate, verifiable ceasefire" and a halt to attacks on civilians.
Libyan leader Muammar Qaddafi's government has made several ceasefire declarations but continued attacks on the besieged western city of Misrata and other rebel-held areas.
Qaddafi has not appeared publicly since April 30, when a NATO air strike on a house in the capital killed his youngest son and three of his grandchildren.
Ban's spokesman Martin Nesirky, asked whether Ban had raised with Mahmoudi whether Qaddafi was alive, told Reuters: "It wasn't discussed...We don't know."
On Egypt and Tunisia – where uprisings led to the ousting of President Hosni Mubarak and Zine al-Abidine Ben Ali earlier this year – Ban said the "revolutions" represented an opportunity to advance democracy and human rights.
"This opportunity is precious but at the same time fragile. It must be nurtured and carefully handled by the people who created it. I have emphasised that this is a once in a generation opportunity so we must assist them."
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