Donald Trump blasted Hillary Clinton Monday for allegedly being too tired to campaign properly.

His remarks came during a pair of dueling Labor Day press conferences that he and Clinton held simultaneously on their separate private jets.

'She didn’t have energy to go to Louisiana and she didn’t have the energy to go to Mexico,' Trump said, when asked about last Wednesday's controversial visit to the Central American country.

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'Low energy': Donald Trump said Monday Hillary Clinton 'didn't have the energy' to visit Mexico or Louisiana like he did and doesn't have the 'stamina' to 'bring jobs back' to the US

'Destruction': Trump also blamed the NAFTA bill, signed in by Bill Clinton, for destroying jobs in the US, and said that he now wants to focus on jobs over immigration

He continued: 'She should have gone to Louisiana and she should have gone to Mexico. She still hasn’t gone to Louisiana and she didn’t have energy or drive to Mexico.'

Trump was referring to both last Wednesday's meeting with Mexican President Enrique Peña Nieto and his visit to flood-stricken Louisiana in mid-August.

He told press after the Nieto meeting that he hadn't discussed his plan to make Mexico pay for a wall separating it from the US - something Hillary Clinton criticized him for Monday.

Clinton's supposed lack of energy came up again when Trump was asked about his message to her for Labor Day.

'Bring back jobs, which she can’t,' he said. 'She doesn’t have clue. Number one she doesn’t have a clue, number two she doesn’t have the energy to bring them back.

'She doesn’t have the energy or stamina to bring them back.'

Clinton has been fighting rumors that she is badly ill, and battled a persistent cough during her speech in Ohio, which later flared up before reporters on the plane.

She reminded them: One thing you know from my doctor's letter is I have seasonal allergies.'

'Allegies': His remarks came the same day that Hillary Clinton suffered a coughing fit, which she attributed to 'seasonal allergies'

Trump was also asked about his stance on immigration, which he said hadn't changed; that America has 'gotta stop the drugs, we gotta stop criminality, get rid of the bad (immigrants), and a lot of them are bad.'

And last Wednesday he unequivocally said that all 11million of the USA's undocumented immigrants would be made to leave the country.

But when pressed by ABC after Monday's flight, Trump seemed less certain about whether all undocumented immigrants would be deported.

When asked whether some might be able to stay, Trump said: 'It could be, but what’s going to happen is if you’re going to be a citizen, you’re going to leave and you're going to have to come back.'

And if they don't want to become citizens, he said, 'they have to make a determination what happens when the border is secure.'

'So you’re open to them staying here undocumented?' Trump was asked.

'I’m going to make a decision, or somebody will,' he replied, 'Whether it’s me or somebody else because by that time we’ll have a secure border, we’ll have a wall.'

During the press event, Trump also accused the NAFTA bill, spearheaded by George HW Bush but signed in by Bill Clinton, of destroying 'the manufacturing and jobs in this country,' and complained about Hillary Clinton's support of the TPP.

Despite his remarks on immigration, Trump said he was making a conscious pivot in his campaign towards focusing on job creation, which he assured the reporters he's 'going to be best at.'

'Everybody knows where I stand on immigration now; I am so about jobs,' he said. 'I see what’s happening in Pennsylvania, I see what’s happening in Ohio and they are getting so wiped out. '

In an interview with ABC Monday, Clinton said that she would not visit Mexico prior to election day, saying she would rather focus on the US.

'I'm going to continue to focus on what we're doing to create jobs here at home, what we're doing to make sure Americans have the best possible opportunities in the future,' she said.