NEW YORK, (AFP) - Hillary Clinton and leftist challenger Bernie Sanders turned up the heat on Thursday in the Democratic race for the White House, locking horns over trade and the "Panama Papers" scandal ahead of the New York primary.

Clinton, the frontrunner and former secretary of state, holds a six-point lead over Sanders in the RealClearPolitics national poll average but lost seven of the last eight voting contests to the Vermont senator.

The New York primary on April 19 has turned into a battle ground, where Clinton needs a commanding win in her adopted home state, which elected her twice to the Senate in 2000 and 2004.

Sanders, who was born and raised in Brooklyn, must capitalise on his recent momentum by winning in New York and in Pennsylvania on April 26 to keep alive his hopes of snatching the Democratic nomination from party favourite Clinton.

The 74-year-old senator sought to capitalise on the leaked Panama Papers, which lay bare how terror groups, drug cartels and pariah countries can hide money in banking havens, by conflating it with Clinton's support for a 2012 Panama free trade agreement.

"I don't think you are qualified if you supported the Panama free trade agreement, something I very strongly opposed, which has made it easier for wealthy people and corporations all over the world to avoid paying taxes owed to their countries," he told a rally in Philadelphia on Wednesday.

Sanders contended at the time that the free trade agreement would make it harder for the United States to crack down on offshore tax havens in Panama.

Clinton, who said on a campaign stop on Wednesday that she would shut down "outrageous tax havens and loopholes" if elected president, helped push through the trade deal through Congress when she was secretary of state.

The former first lady has repeatedly questioned whether Sanders is a qualified commander-in-chief when the 74-year-old Senator has served his entire career in the small state of Vermont, first as mayor then as representative in Congress.

"It is all about rolling up our sleeves and getting to work. Don't make promises you can't keep," she told reporters on Thursday while campaigning in the Bronx, where she rode the subway joined by a local Democrat politician.

"Know what you want to achieve and then bring everybody together to get the results and that is what I'm going to do."

New York, America's largest city and one of its most diverse, has demographics that play well to Clinton's support base among the wealthy and minorities, but observers warn there may be tougher terrain outside the city.

Communities in the more economically hard-hit western and northern part of the state voted Clinton into the Senate, but critics say they have not seen the job growth they were expecting.

Sanders has resonated strongly among voters, particularly independents, for his steadfast opposition the trans-Pacific trade deal signed by President Barack Obama that Clinton has only opposed more recently.

"If the Clinton campaign wants to engage in a more bare-knuckled kind of approach, we're happy to do that as well," Sanders's campaign director Jeff Weaver told MSNBC on Thursday.

"Secretary Clinton is funded by Wall Street interests and other special interests. You know, she's really made a deal with the devil, and the devil always wants his due. So that time will come," he said.

In Philadelphia on Wednesday, Clinton criticised Sanders, saying "in a number of important areas, he doesn't have a plan at all."

This week she also ripped into her opponent for saying in a newspaper interview that he did not agree with efforts by parents of children killed in a shooting at a Connecticut elementary school to sue gun manufacturers.

"That he would place gun manufacturers' rights and immunity from liability against the parents of the children killed at Sandy Hook is just unimaginable to me," she told MSNBC on Wednesday.

Clinton and Sanders also sparred for days about holding a debate ahead of the New York primary - now scheduled for April 14 in Brooklyn, the borough where Sanders was born and which Clinton made her national campaign headquarters.

Clinton leads Sanders 54-42 per cent among likely Democratic voters in New York and 50-44 per cent in Pennsylvania, according to Quinnipiac University polls.