By James B. Kelleher

CHICAGO (Reuters) - The Chicago Teachers Union, which has vowed to strike next week without a new contract, filed unfair labor practice charges against the Chicago school system on Wednesday, ratcheting up pressure on Mayor Rahm Emanuel to make concessions as a key deadline looms.

The union also said on Wednesday it would not extend the Monday strike deadline if no contract agreement is reached with the city.

A strike by the nearly 30,000 public school teachers and support staff in the nation's third-largest school district would be the first in Chicago in 25 years and one of the largest labor actions nationwide in recent years.

The union filed complaints with the state labor board accusing the school district of violating state law by unilaterally imposing changes in teachers' working conditions - including new teacher evaluation procedures - while both parties are still at the bargaining table.

The union also charges that Chicago Public Schools, which has more than 400,000 students enrolled, is refusing to allow arbitration on some complaints and has intimidated teachers picketing at a school.

Chicago Public Schools spokeswoman Becky Carroll said the district was disappointed, and called on the union to focus its energy on the contract talks.

"It's time to put antics aside and negotiate in good faith," Carroll said.

Unless the two sides can agree to a new contract, the union said it will strike at 12:01 a.m. on Monday, the second week of classes for most students. Both the school district and parents are scrambling to come up with contingency plans to cope with possible disruption.

Chicago Mayor Emanuel, a former White House chief of staff to President Barack Obama and a speaker at the Democratic National Convention this week, has made reform of the city's troubled public schools a top priority of his administration. Earlier this year, he succeeded in pushing through a longer school day.

But the union is opposed to other proposed reforms backed by Emanuel, including tougher teacher evaluations tied to student test scores and giving principals wide latitude in hiring.

The union also wants a larger pay increase for teachers than the 8 percent raise over four years that Chicago is offering.

Jean-Claude Brizard, Emanuel's school system chief executive, has said that the district cannot afford the raises demanded by the union because of a projected $3 billion deficit over the next three years.

The city of Chicago and the state of Illinois are in dire financial straits and credit ratings for both the city and state have been downgraded. (Editing by Greg McCune and Lisa Shumaker)