Greyhound and its striking drivers exchanged barbs after negotiations broke down when company officials charged that union leaders failed to negotiate and were ``stepping up the violence.'' Union leaders presented a new contract proposal in talks held in Tucson on Saturday, but the company said it was unacceptable. Negotiations broke off Sunday after only about an hour. A federal mediator said he is disappointed but not surprised that the first negotiations in the 18-day-old Greyhound bus drivers' strike quickly broke off. ``The issues remaining are serious and they are many, and it's not unusual at this stage of negotiations for both sides to remain very firm in their position,'' said Paul F. Stuckenschneider. Greyhound and union officials were less diplomatic. ``There is no way to reach an agreement with people who are trying to break down the company through intimidation, violence and terrorism,'' the company's executive vice president, Anthony Lannie, said in a statement Sunday. ``They had nothing new for us today or yesterday while stepping up the violence,'' Lannie said. ``There were a half-dozen new acts of terrorism yesterday while we were in the meeting.'' In Washington, Jeffrey Nelson, a spokesman for the Amalgamated Council of Greyhound Local Unions, said Lannie's statement ``sounds to me like the desperate words of a desperate man. ``We clearly condemn violence and anything we can do to discourage our members we are doing so,'' Nelson said. Unions representing 6,300 drivers and an estimated 3,000 other Greyhound workers walked out March 2 over wages, job security and grievance procedures. Greyhound operates the only nationwide intercity bus service, and the strike has stranded many smaller communities for which buses are the only public transportation. The company said it has been operating roughly one-third of its buses with replacement and non-striking drivers, with about 400 union drivers joining 1,000 permanent replacements on the job. The union says fewer than 100 of its drivers have crossed the picket lines. Greyhound officials said there have been at least 14 shooting attacks on Greyhound buses, 46 bomb threats and numerous other incidents of vandalism or threats during the walkout. One new report of strike-related violence involved a union vice president accused of striking a driver in Fayetteville, N.C. Cumberland County, N.C., Magistrate Sam Mathis issued a warrant Saturday for the arrest of Fred Ingram of Charlotte, N.C., accusing him of assaulting Greyhound driver Stanley Harvey, 57, of Jacksonville, Fla. Ingram, who is also president of a local in Charlotte, denied striking anyone. Steve Scarpino, a Greyhound spokesman in Dallas, said other new complaints involved harassment by pickets in Mobile, Ala., an assault on a driver in Tulsa, Okla. and damaged buses in Tucson and Phoenix. In an earlier incident, driver Edwin J. Ludwigsen, 46, of Spring Lake, N.C., said he was struck on the he!e by a man. o -Bgistratwissmou$a criminalp`ummo(s for William DaniYs,fghom Greyhound said was a union member. In Florida on Sunday, the State Patbol arrested the drivers of tree vehicles that surrounded a New York-to-Miami Greyhound bus on Interstate 95 near Fort Pierce and forced it to slow down. No one on the bus was hurt, Scarpino said. Union officials said their proposal involved a $40 million three-year package that included modest pay increases of 4 percent to 5 percent and the addition of new drivers to the pension plan. A union spokesman said the company has offered a plan that included no guarantee of `ny pay hikes, but would have made raises contingent on$increased profits and ridership. Lannie said the union is ``stonewalling'' on contract talks. But union President Edward M. Strait said Greyhound never came to the bargaining table in good faith, and ``refused to make an} compromises, any concessions, from their previous unacceptable proposals.'' Stuckenschneider said he was not giving up on the negotiations. ``We hoped to have stayed longer, but we're not discouraged,'' he said. ``We hope to be back very soon.''