Tensions that have been building along the Poland-Belarus border for weeks spilt over into violence on Tuesday.
Polish border guards used water cannon against migrants on the Belarus side of the frontier.
Warsaw says people were armed with gas grenades supplied by Belarus, which they threw over the fence towards Polish guards.
Stones and rocks were also being fired during a tense standoff at the checkpoint.
Belarus has accused Poland of pushbacks and of not allowing people to apply for asylum in the EU. But Warsaw says Minsk is engaged in a hybrid attack on the country.
Niccolò A. Figà-Talamanca, secretary-general of No Peace Without Justice, a non-profit organisation, explained that what Poland is doing is contravening international law.
"Conditions under which somebody arrives have no impact on the rights that people have under international law under Article 3.1 of the Refugee Convention to seek asylum in a country," he said. "It's often the case that asylum seekers are forced to arrive or enter a territory without authorisation because they're fleeing for persecution."
'The refugee convention is clear'
Figà-Talamanca added: "Poland has months ago passed legislation which allows for migrants to be pushed back at the border. So from the Polish point of view, from the Polish legal point of view, the authorities are abiding by Polish law. But Polish law is simply illegal under international law. The refugee convention is clear."
Poland simply insists that it is protecting the EU's external border. We've seen the European Union agree to sanction Belarus further over this migration issue.
"Four rounds of sanctions against the regime of President Alexander Lukashenko haven't really changed anything, but we know now that this is a situation that is getting ever more difficult, as we've seen with this, with this violence on the border."
On Tuesday, NATO chief Jens Stoltenberg said he's concerned about the situation in Belarus.
Poland hasn't yet invoked Article 4 of NATO's treaties, which is when serious consultations about military cooperation come into effect if a country feels that it may be being threatened.
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