BEIJING, Jan. 20 (Xinhua) -- The United States is witnessing a government shutdown, the fourth over the past 25 years, as the Senate failed to pass a stopgap spending bill.

Democratic lawmakers have demanded that a spending deal should include protections for young immigrants. But Republicans who control both chambers of the Congress want to separate immigration reform from the budget deal discussions.

Negotiations between congressional lawmakers continue.

U.S. Congress used to follow a procedure of passing a budget first, then 12 separate appropriations bills. That process has been paralyzed in recent years because of increased political brinkmanship, and the Congress turns to a stopgap continuing resolution, or CR, that maintains federal government spending at existing levels for all or part of the fiscal year.

A shutdown doesn't mean the whole government will come to a halt. Spending for essential functions related to national security and public safety will continue, such as payment for U.S. military troops, while federal employees of non-essential services will face being furloughed. There is no assurance of receiving retroactive payment for furlough days once the shutdown crisis is over.

The following is a look at recent U.S. government shutdowns:

-- October 2013. Sixteen-day partial shutdown came as Congress failed to come to an agreement on a budget after Republican lawmakers began pushing to defund Democratic President Barack Obama's Affordable Care Act, known as Obamacare. Senate Democrats and the Obama administration rejected the proposals and the resulting impasse led to the partial shutdown.

The shutdown affected most government operations and resulted in the furlough of about 800,000 federal employees, according to a report by the Congressional Research Service.

-- Late 1995 and early 1996. The Bill Clinton administration and the House of Representatives failed to agree on a budget to fund the government. The two shutdowns last for 28 days in total, until a new resolution was finally reached.

Since 1976, when the current budget and appropriations process was enacted in the United States, there have been 18 gaps in budget funding, according to the Congressional Research Service.