By Louis Charbonneau

UNITED NATIONS, June 29 (Reuters) - The United Nations envoy to Syria told the U.N. Security Council on Wednesday that it remains unclear when the next round of U.N.-brokered peace talks will take place, adding that there was no point in talking without some assurance of progress.

The last round of talks between the Syrian government and opposition broke up at the end of April as government forces, backed by Russia, escalated their assault on rebel-held areas in the northern city of Aleppo.

"I have not ... indicated a fixed date in July," Staffan de Mistura told reporters after briefing the 15-nation Security Council on his work to secure a negotiated solution to the five-year-old Syrian civil war. "I'm still aiming within July, but not at any cost and not without some guarantees."

"When you convene a conference or talks ... you want to make sure that it has good chances of success," he said. "Having just a conference for the sake of a conference, we can do that anytime. We can do it tomorrow, if you want."

A "cessation of hostilities" that had brought peace to much of Syria for two months has largely broken down, and the war has resumed in many areas.

The negotiations on a political transition center on the future of President Bashar al-Assad, who is supported by Russia and Iran but who Western and Gulf Arab governments would like to see replaced.

British Ambassador Matthew Rycroft told reporters that the conditions were not there for a new round of U.N.-brokered talks between the Syrian government and opposition.

"There continues to be such a breach of the cessation of hostilities, such a lack of humanitarian access, that it's very hard to see how the conditions can be arrived at for political talks to resume," he said. "That is such a tragedy, above all for the people of Syria."

De Mistura suggested he has not given up on an August deadline for the Syrian parties to present the outlines of a political deal, though Security Council diplomats say it is a deadline that will almost certainly be missed.

"What we need is that the stakeholders do come with a feeling of urgency and work on some ideas on how to bridge the differences," he said.

Despite some increases in humanitarian aid access inside Syria, the United Nations says that hundreds of thousands of civilians in Syria need food and medicine.

(Reporting by Louis Charbonneau; Editing by Andrew Hay) ((louis.charbonneau@thomsonreuters.com; +1-212-355-6053; Reuters Messaging: louis.charbonneau.reuters.com@reuters.net; Twitter: @lou_reuters))