The Chicago Teachers Union and the nation's third largest school system have reached an "agreement in principle" to end a five-day strike over Mayor Rahm Emanuel's demand for education reforms, officials say.

The tentative deal, announced on Friday, raises hopes that teachers would be back in class soon.

"CPS (Chicago Public Schools) and CTU (Chicago Teachers Union) have come to an agreement in principle," the school district said on Twitter.

David Vitale, Chicago School Board president, said the framework deal should allow students to be back in school on
Monday morning.

More than 350,000 Chicago students marked a week off classes on Friday after about 29,000 Chicago public school teachers and support staff went on strike over the education reforms.

The teachers walked out last Monday in the first Chicago Teachers Union strike since 1987. It was the largest strike in the US in a year.

A tentative deal

Talks are set to continue through the weekend to put the framework deal into legal terms so that teachers union delegates could decide on Sunday whether or not to suspend the strikes, negotiators said.

They added that the agreement in principle addressed all issues.

Details of the tentative agreement were not immediately released, but it seemed that Emanuel had retreated on his teacher evaluation demands, agreeing to reduce the importance of standardised tests.

Chicago teachers have argued that tying teacher evaluations to students' performance on standardised tests in areas such as reading and math forces them to teach to the test and narrows the curriculum.

They also said they should not be evaluated on factors outside their control such as poverty and crime, which plague many of their students.

The deal is expected to be costly for Emanuel and the school district, which is facing a financial crisis and $665m budget deficit this year.