Hong Kong democracy activists kicked off new round of protests that began over a plan to allow extraditions to mainland China.
The officers added that they are unsure whether they would be informed ahead of time if Chinese paramilitary or army forces were deployed in Hong Kong.
Trump has been reticent to criticize China's stance on Hong Kong, indicating he regards the unrest as an internal matter for Beijing.
It was the second time this week that China had sought to frame the protests as 'terrorism' which have raised fears it may deploy force to quell the unrest.
Protesters circulated letters and promotional materials apologizing to travellers and the general public for inconveniences during the past five days of airport.
Hong Kong has been gripped by ten weeks of protests that have seen millions take to the streets, sparked by opposition to a planned law that would have allowed extraditions to mainland China.
Chinese state and social media had earlier aired video of security forces gathering across the border from the enclave.
Under the 'One country, two systems' principle, the media in Hong Kong enjoys freedoms unheard of in mainland China.
US President Donald Trump has so far been muted on the growing unrest in Hong Kong, saying it is an internal affair for China.
Authorities said more than 5,000 people descended on the Hong Kong airport carrying placards and chanting slogans about police violence.
Tear gas was also deployed in central Hong Kong, in the Tsim Sha Tsui area on the Kowloon side and in Wan Chai on Hong Kong Island.
The pro-democracy protests, which are partly fuelled by widespread anger at an erosion of liberties in Hong Kong, have become the biggest threat to Beijing's rule.
The protests have been propelled by an underlying distrust for the ruling Communist Party on the mainland, where speech is tightly controlled and dissenters are routinely jailed.
Angry commuters argued with protesters in a stopped train and on the platform at Tiu Keng Leng station.
Pro-democracy protesters fought a second consecutive day of running battles with police in a well-heeled residential district of Hong Kong.