* Crisis focused on delayed release of jailed Palestinians
* Kerry cancels planned face-to-face talks with Abbas
* Peace talks facing April 29 deadline (Adds PLO official's comments)
By Noah Browning
RAMALLAH, West Bank, April 2 (Reuters) - A surprise decision by President Mahmoud Abbas to sign more than a dozen international conventions that could give Palestinians greater leverage against Israel left the United States struggling on Wednesday to put peace talks back on track.
A senior Palestinian official, voicing frustration deepened by Israel's failure to carry out a pledged release of several dozen Palestinian prisoners, said the eight-month-old talks had become merely "negotiating about negotiating".
The Palestinians on Wednesday handed over to a U.N. representative and other diplomats applications Abbas signed late on Tuesday to join 15 international conventions.
They include the Geneva Conventions, the key text of international law on the conduct of war and occupation.
Palestinian officials said Israel's failure to let the prisoners go meant Abbas no longer had to stick to a commitment not to confront it at the U.N. and other international bodies.
The developments further complicated efforts by U.S. Secretary of State John Kerry to piece together a three-way deal to push the faltering negotiations past an April 29 deadline and into 2015.
The talks were already in trouble over the issues of Israeli settlements in the West Bank and East Jerusalem - land captured by Israel in the 1967 Middle East War - and Palestinian opposition to Netanyahu's demand to recognise Israel as a Jewish state.
Israel had said it first wanted a Palestinian commitment to negotiate past the original target date for a deal before freeing the last of the 104 prisoners it promised to release as part of U.S. efforts to restart the negotiations last July.
"UP TO THE PARTIES"
In Brussels for a NATO meeting, Kerry cancelled a planned visit to the West Bank city of Ramallah on Wednesday to meet Abbas, saying it was important to keep the peace process moving but "in the end, it is up to the parties".
Palestinians hope Abbas's move will give them a stronger basis to appeal to the International Criminal Court and eventually lodge formal complaints against Israel for its continued occupation of territory seized in 1967, lands they see as vital to an independent state. Most countries deem the Israeli settlements as illegal.
A spokesman for Israeli Prime Minister Benjamin Netanyahu declined to comment on Abbas's move.
Yasser Abed Rabbo, deputy head of the PLO, cautioned on Wednesday against simply returning to an "empty routine" at the negotiating table. He reaffirmed that Palestinians wanted talks to focus on setting the future borders of their state.
"We can't return to the empty routine, a search for a framework for talks - this empty routine which is negotiating about negotiating," he told reporters.
Continuing the talks beyond the end of this month, he said, "must proceed from and depend on one main point, and this is looking into the issue of borders."
The conventions signed by Abbas were mostly sets of international standards on social and rights issues, such as a conventions against discrimination against women and for the rights of disabled people as well as the Geneva Conventions.
George Giacaman, a political science professor at Birzeit University in the West Bank, said Abbas wanted "to show that he's serious, and not just he, but the entire PA (Palestinian Authority) and the PLO must be taken seriously".
Law professor Robbie Sabel, a former legal adviser to the Israeli Foreign Ministry, described Abbas's signature on the conventions as "merely symbolic". He noted Abbas had stopped short of applying for membership in international organisations.
Kerry made an unscheduled visit to Jerusalem on Monday seeking to extend the negotiations by putting together a proposal that included the possible release of Jonathan Pollard, an Israeli spy jailed in the United States in the 1980s.
The package, officials close to the talks said, included an additional Israeli release of hundreds of jailed Palestinians and a possible partial freeze on the settlements.
Pollard, a U.S. citizen and former navy analyst, is serving a life term for spying for Israel. His freedom would be a political triumph for Netanyahu, making it easier for him to sell a wider release of jailed Palestinians to cabinet members and a sceptical Israeli public. (Additional reporting by Ali Sawafta in Ramallah, Lesley Wroughton in Brussels and Maayan Lubell in Jerusalem, Editing by Jeffrey Heller and Angus MacSwan)