The Carter administration is planning a series of modest new job-creation measures as part of the overall tax-cut and economic stimulus program the President will propose to Congress later this month.

Although details haven't been ironed out yet, the package includes continuation and expansion of existing government-subsidized jobs and revamping some tax credits to encourage hiring of the hard-core unemployed.

In addition, thhthte White House is hoping to finish drafting its new "urban policy" proposals by early spring, in time to expand its January budget targets.

The proposals, to be combined with the President's $25 billion tax-reduction package, are intended to defuse criticism from blacks and liberals that the administration is not doing enough to spur new jobs.

Combined with a variety of other increased outlays, the job measures have pushed the administration's total spending target for fiscal 1979 to just over $500 billion - a few hundred million dollars over what Carter intended.

The proposals include:

Expansion of the existing public works jobs program by about $500 million - enough to create 20,000 new jobs - primarily to hire persons who have completed tours under the public-service jobs program.

Extension of the public-service jobs program for at least one year beyond its present Oct. 1 cutoff, with a gradual phaseout in future years in line with a reduction in the jobless rate.

Continuation beyond this calendar year of the tax credit Congress passed in 1977 to encourage employers to increase their payrolls - but with a special proviso earmarking the breaks for those who hire hard-core unemployed.

Inclusion of a new pilot program in the emerging welfare revision proposal that would provide 30,000 to 60,000 new job slots for hard-to-employ persons who cannot find work in the private sector.

At the same time, the Treasury is reported to be exploring new tax breaks for small business to be included in Carter's coming tax-reduction package as a way to relieve pressure on small- and medium-sized firms. Sources say the White House hasn't yet decided on the size and scope of the small-business proposals. However, tax planners have received high-level, clearance to look into several alternatives.

Although the job proposals would be modest, officials assert they would enable the administration to counter some recent criticism that it is not doing enough to reduce unemployment in inner cities.

The jobless rate now is 6.9 per cent of the work force. Economists say that even with Carter's proposed $25 billion tax-cut package, the unemployment rate will not fall below 6.4 per cent this year.

The $500 billion budget total would produce an overall fiscal 1979 deficit of about $59 billion - about the same as the $58.5 billion red-ink figure now estimated for the current fiscal year.

The administration had hopes to keep spending below the symbolic $500 billion mark next year and also to reduce the deficit, but increases in defense spending - and inflation - pushed the total over.

The $500 million increase in the public-works jobs program would be a relatively modest addition to a plan that allots federal money to state and local governments to finance construction projects built by local firms.

Existing public service jobs total about 725,000. After the one-year extension, this would be trimmed by 100,000 jobs for each .5 per cent drop in the jobless rate.

Carter is expected to unveil his-tax cut and spending proposals in a series of speeches and messages during the final two weeks of this month. His State of the Union address also is expected to deal primarily with pocketbook issues.