The US Treasury has begun taking “extraordinary measures” to meet its obligations, after the US government hit its $31.4tn borrowing limit, Janet Yellen said in letter to congressional leaders on Thursday as she urged lawmakers to raise the debt ceiling sooner rather than later to avoid an unprecedented default.

Yellen, the Treasury secretary, had previously warned that the US government would run up against the debt ceiling — the country’s legal borrowing limit — on Thursday. Yellen sent the letter confirming the breach to Republican Speaker of the House Kevin McCarthy and Democratic House minority leader Hakeem Jeffries on Thursday morning.

Yellen cautioned that there was “considerable uncertainty” surrounding how long the “extraordinary measures” could continue before the US government risked default.