Negotiations to stave off another government shutdown have reached the stage of technical discussions over ways to raise extra government revenue after “good progress” was made between Washington lawmakers aiming at $1tn budget deal.

Congressional staff were expected to hold talks on Thursday to thrash out finer details, according to sources close to the process, after a brief, but productive, meeting in private on Wednesday between Democratic senator Patty Murray and Republican Paul Ryan.

The Democratic chair of the Senate budget committee is leading formal negotiations with her Republican counterpart in the House of Representatives that hold out the best hope of avoiding another crippling government shutdown and ending months of stalemate.

But the Congressional calendar is adding to the long list of political obstacles that have previously stood in the way of reaching this crucial agreement on next year's federal budget.

House and Senate sessions only overlap for five days next week before the lower chamber breaks up for Christmas, making the next few days the last practical chance to strike a deal before current spending authority expires on January 15.

The two interlocutors met for half an hour on Wednesday after Murray's return from Thanksgiving for the first in a series of crunch-time talks aimed at meeting a self-imposed deadline to present a proposal by December 13.

There are no new face-to-face talks scheduled for Thursday or Friday but they may hold further discussions by phone over the weekend if progress is made between staff negotiators.

Sources close to the negotiations say they are within grasp of a two-year deal that would set the annual government budget at approximately $1tn – roughly half way between the $1,058tn sought by Democrats and the existing $967bn that Republicans would like to stick to.

Aides are keeping many of the politically-sensitive details under wraps for now, but the Guardian understands they are also discussing limited relief from the sequester, which forced government agencies to apply blanket budget cuts after negotiations last Christmas failed to reach any compromise that would allow more targeted reforms.

This time around it looks like some limited spending increases will be made possible by offering alternative ways to raise government revenue rather than the tax rises that Republicans are so strongly opposed to.

Such a strategy would require a tricky political balancing act and leaders of both parties have been downplaying expectations in recent days for fear of triggering a backlash from those on either wing who would regard such a deal as a sell-out.

Moderate Republicans are more optimistic however and have suggested one such revenue-raising route might involve demanding that federal workers contribute more toward their pensions.

Senator Patrick Toomey for example, who has previously sided with Democrats on issues like gun control and gay rights, suggested on Wednesday that he would be back such a compromise. “The chances [of a budget deal] are better than even,” he told MSNBC.

But there are plenty of left-leaning Democrats who are wary of such moves, and have fought off separate plans to cut entitlement programmes such as social security that were proposed by the White House in its suggested budget compromise.

With hardliners on both sides reluctant to give ground on taxes or entitlement cuts, the contours of the deal sought by Murray and Ryan are likely to remain limited.

Nevertheless, any deal at all would be a major achievement for a Congress that is poised to strike a new record for the least legislation passed in one year and would be a much-needed confidence boost for the economy.