Republican presidential front-runner Donald Trump on Friday criticised his party's rules which allegedly allow candidates to win over delegates through unethical means like offering plane tickets or hotel rooms, saying this "phony, crooked" delegate selection process in not democratic.

"I did know it and it's troubling -- not only troubling, it should be illegal. You shouldn't be able to do it," Trump said when asked about the little known rule of the Republican party, which may come into play in a big way at a time when none of the three presidential candidates are likely to get the requisite 1,237 delegate's support before the July convention in Cleveland.

Trump currently is leading the delegate count but is still far away from the 1,237 number. His two rivals Senators Ted Cruz from Texas and Governor John Kasich from Ohio are banking on the fact that they would win over the delegates at the convention.

"We are fighting for delegates, and what they're doing is take a look. They're travelling them around. They're taking them out to dinner. You could actually take a couple of hundred million dollars and buy an election and never win a race because all you have to do is take them out to dinner and send them to Paris, France, for the evening," Trump told Fox News.

"I think it's a disgrace and that it is not our system and this should not be the Republican Party. I'm so proud of myself, I exposed this. This was going on for a long time and it's now been exposed, and it shouldn't be allowed to be," the real-estate tycoon said.

Claiming that Kasich has no path to nomination, Trump said, "It's mathematically impossible for John Kasich to become the Republican nominee. He needs more than 100 per cent of the remaining delegates."

"It's worth remembering Kasich went for 0 for 27, lost 27 states in a row. Then he won his home state. You can't lose every state and expect to be the nominee. Right now, Kasich's role is really being a spoiler," he said.

Similarly, he argued that Cruz also has no path to nomination. "He's mathematically eliminated. He's out. And they should both drop out," Trump said. "Remember this. While I'm almost 300 delegates ahead and millions of votes ahead, it's really unfair because I wasn't competing against one person, I was competing against 16 people at the beginning. So we'd have splits with 16 people, and then 14 people and 12 people and eight people and nine people. And you know, it would go on that way, and then even now, we have three people," he noted.

"We should be having, like, just one. I don't really care and we're winning by a lot but it really isn't fair because when you break it up, when you have that many people, you're getting much less. As an example, Marco Rubio has more delegates right now than Kasich, and those are delegates that are taken out of play, so it makes it hard for me to get them. And he's right now not in play," Trump said.

"So the whole system is rigged. It's a phony, crooked system where people are allowed to buy delegates, they're allowed to buy hotel rooms for them and take them out to dinner and do whatever they have to do to get their vote. That's not democracy," Trump said.