WASHINGTON, May 25 (Reuters) - The chances of reviving the 2015 Iran nuclear deal are shaky at best and Washington is ready to tighten sanctions on Tehran and respond to "any Iranian escalation" with Israel and other allies if it cannot be saved, the United States' Iran envoy said on Wednesday.

"We do not have a deal with Iran and prospects for reaching one are, at best, tenuous," U.S. Special Envoy for Iran Rob Malley said in prepared congressional testimony, saying that if the accord could not be resurrected, the United States was "ready to continue to enforce and further tighten our sanctions ... and to respond strongly to any Iranian escalation, working in concert with Israel and our regional partners." (Reporting By Arshad Mohammed, Humeyra Pamuk and Patricia Zengerle; editing by Jonathan Oatis)