WARSAW, Sept 20 (Reuters) - Three people have died after crossing into Poland from Belarus, and a fourth person has been found dead on the Belarusian side of the border following a surge in illegal migration that has caused a diplomatic row. Polish officials, who announced the three deaths inside Poland, gave no cause of death on Monday. The Belarusian border service identified the person found inside Belarus as "a woman of non-Slavic appearance" but did not say how she died. Polish Prime Minister Mateusz Morawiecki said there had been almost 4,000 attempts to illegally cross Poland"s border in September and nearly 7,000 over the last two months, and put the blame for the increase in migration on Minsk. "(Belarus) has introduced a visa-free regime with several countries with great potential for illegal immigration, tens of thousands of people are being brought to Belarus," he said. The European Union has also accused Belarus of encouraging people to cross into Polish territory, many of them from countries such as Iraq and Afghanistan, to put pressure on the bloc over sanctions it has imposed on Minsk. Poland has started building a barbed wire fence to curb the flow of migrants and has declared a 30-day state of emergency in a three-km (two-mile) deep strip of land along the frontier. read more . "There were three people who died unfortunately on the Polish side, as far as we know on the Belarusian side it was one woman," Polish border guard chief Tomasz Praga said. Although he did not say how they died, he said there had been many cases of people being exhausted or suffering from hypothermia after arriving from Belarus, and some had been saved from drowning in swamps. The Belarusian border service said "clear signs of dragging a body from Poland to the Republic of Belarus were recorded" near the corpse found on the Belarusian side of the frontier. "This is a lie," Praga said. The Belarusian interior ministry declined to comment to Reuters. The Belarusian border service was not immediately available to comment. Praga said 110 people had been detained for allegedly helping organise illegal crossings. A pregnant woman had been detained after crossing the border with 13 children, none of them her own, and the children were admitted to hospital with COVID-19, he said. Interior Minister Mariusz Kaminski said 500 more soldiers would be deployed to help the border guard protect the frontier.