Philippine President Rodrigo Duterte said Monday he may turn down an invitation by Donald Trump to visit the United States, as he welcomed three Chinese warships to his home town. Duterte, who has loosened the Philippines' long alliance with the United States while strengthening ties with China and Russia, said he could not commit to the American president because of a busy schedule that included a trip to Moscow. "I am tied up. I cannot make any definite promise. I am supposed to go to Russia, I am supposed to go to Israel," he told reporters when asked about Trump's invitation made in a telephone call on Saturday. Duterte expressed concerns about not being able to fit in a visit to Trump even though no firm date has yet been proposed for it. Nevertheless, Duterte said relations with the United States were improving now that Trump had taken over from Barack Obama, who criticised the Philippine president for his anti-drug war that has claimed thousands of lives. Rights groups have warned Duterte may be orchestrating a crime against humanity, with police and vigilantes committing mass murder. But Duterte insists his security forces are not breaking any laws. Duterte last year branded Obama a "son of a whore" in response to the criticism. He also declared while in Beijing last year that the Philippines had "separated" from the United States. The United States is the Philippines' former colonial ruler and the nations are bound by a mutual defence treaty. Duterte said Monday that his efforts to loosen the alliance were only a response to the drug war criticism. "It was not a distancing (of relations) but it was rather a rift between me and the (US) State Department and Mr Obama, who spoke openly against me," he said. "Things have changed, there is a new leadership. He wants to make friends, he says we are friends so why should we pick a fight?" - 'Confidence building' - Duterte's comments came shortly after he visited three Chinese warships visiting his home town, the southern city of Davao on Mindanao island. "This is part of confidence-building and goodwill and to show we are friends and that is why I welcome them," he said. Duterte has pursued closer relations with the Chinese government even though Beijing has taken control of a fishing shoal and built artificial islands in parts of the South China Sea that are within the Philippines' exclusive economic zone. China claims nearly all of the strategically vital waterway, even waters approaching the coasts of its neighbours. Vietnam, Brunei, Malaysia and Taiwan also have claims in the sea. China's expansionism in the waters have triggered concern regionally and in the West, with its new artificial islands capable of serving as military bases. But on Sunday Duterte issued a chairman's statement, after hosting a 10-nation Association of Southeast Asian Nations summit, which took a soft stance towards Chinese actions in the sea. The statement merely took note of "concerns expressed by some leaders over recent developments in the area". It ignored an international tribunal ruling last year which said China's claims to most of the sea were unlawful. It also did not carry wording from previous ASEAN statements calling for a "respect for legal and diplomatic processes" in resolving the dispute. Duterte has repeatedly said the Philippines and other nations are helpless to stop the island-building, so there is no point challenging China in diplomatic and legal circles. He has instead promoted what he says will be billions of dollars' worth of investments from China that he expects will result from the improvement in bilateral relations. Duterte on Monday also repeated that he was open to joint military exercises between the Philippines and China. "I said I agree. There can be joint exercises," said Duterte, who has scaled back regular war games with the United States.